refactor(bw-report): parse user notes by POSITION, not by label

The four operator-supplied note fields in BW's Compliance Setup →
Notes tab (Project / Client / User Name / Seis Loc) have
USER-EDITABLE LABELS — an operator can rename them in BW's UI to
"Building:", "Site Address:", "Inspector:", or anything else, and
the ASCII export writes those literal labels verbatim.  The
previous label-normalisation map approach (just added in commit
6a7e8c6) was fragile: it could only match label spellings we'd
enumerated in advance.  An operator using "Site:" instead of
"Seis Loc:" would have their sensor location silently dropped.

What IS reliable: BW always writes the 4 user-notes lines
contiguously, in the same order, between the "Units :" line and
the "Geo Range :" line of the export.  So parse them by POSITION:

  position 1 → project
  position 2 → client
  position 3 → operator
  position 4 → sensor_location

The original labels BW wrote are preserved in a new
`BwAsciiReport.user_note_labels` dict (canonical slot → literal
label string) so terra-view can render them as the operator named
them.

Removes the `_OPERATOR_LABEL_MAP` / `_normalise_label_for_lookup`
helpers and the elif-by-normalised-label branch in `parse_report`.
Replaces with a small state machine that flips on the "Units" line
and flips off on the "Geo Range" line.

Tests:
  - Default-label fixtures (waveform + histogram) still populate
    correctly, with operator's labels captured.
  - Synthetic custom-labelled exports ("Building:" / "Site Address:" /
    etc.) populate the right slots by position.
  - Histogram-specific "Seis. Location:" works.
  - Lines outside the Units→Geo Range range are ignored even if
    they look like user notes (defensive against malformed exports).
  - Partial blocks (fewer than 4 lines) leave later slots None.
  - Extra lines beyond 4 are dropped (5th slot doesn't exist).

26 tests in test_bw_ascii_report.py (was 33; net drop reflects
parametrised label tests collapsed into 6 focused position tests).
Full SFM suite: 62 passed, 44 skipped.

Pairs with series3-watcher v1.5.0 which fixes the filename pairing
so the report reaches this parser in the first place.

tests/test_bw_ascii_report.py

@@ -259,53 +259,135 @@ def test_parse_handles_micl_double_space_in_key():
     assert r.mic.zc_freq_hz     == pytest.approx(51.0)
 
 
-# ── Operator-field label normalisation ──────────────────────────────────────
+# ── Position-based user-notes parsing ───────────────────────────────────────
+#
+# The 4 user-supplied note slots (Project / Client / User Name / Seis Loc
+# by default) have OPERATOR-EDITABLE labels in BW's Compliance Setup →
+# Notes tab.  An operator could rename them to "Building:", "Site:",
+# "Address:", etc. and the ASCII export would write those labels
+# verbatim.  We parse by POSITION between the `Units :` and `Geo Range :`
+# anchors, NOT by matching the label text.
 
 
-@pytest.mark.parametrize("label,expected_field,expected_value", [
-    # project — both with-colon and bare
-    ("Project:", "project", "Test4-21-26"),
-    ("Project",  "project", "Test4-21-26"),
-    # client
-    ("Client:",  "client",  "Acme Inc"),
-    ("Client",   "client",  "Acme Inc"),
-    # operator (User Name)
-    ("User Name:", "operator", "Brian"),
-    ("User Name",  "operator", "Brian"),
-    # sensor_location — every Seis*/Sensor Location variant we know about
-    ("Seis Loc:",       "sensor_location", "Catbed"),
-    ("Seis Loc",        "sensor_location", "Catbed"),
-    ("Seis. Location",  "sensor_location", "Catbed"),
-    ("Seis Location",   "sensor_location", "Catbed"),
-    ("Sensor Location", "sensor_location", "Catbed"),
-])
-def test_operator_label_variants_route_to_correct_field(label, expected_field, expected_value):
-    """All known label spellings of the operator-supplied fields route
-    to the same dataclass attribute."""
-    text = '"{} : {}"\n'.format(label, expected_value)
+def _wrap_user_notes(*lines: str) -> str:
+    """Helper: wrap N user-note lines in the minimal context the parser
+    needs (`Units :` opens the block, `Geo Range :` closes it)."""
+    body = ['"Units : in/s and dB(L)"']
+    body.extend('"' + l + '"' for l in lines)
+    body.append('"Geo Range : 10.000 in/s"')
+    return "\n".join(body) + "\n"
+
+
+def test_user_notes_default_labels_populate_by_position():
+    """The BW-default labels (Project / Client / User Name / Seis Loc)
+    populate the four canonical slots in order."""
+    r = parse_report(_wrap_user_notes(
+        "Project: : Test4-21-26",
+        "Client: : Acme Inc",
+        "User Name: : Brian",
+        "Seis Loc: : Catbed",
+    ))
+    assert r.project          == "Test4-21-26"
+    assert r.client           == "Acme Inc"
+    assert r.operator         == "Brian"
+    assert r.sensor_location  == "Catbed"
+    assert r.user_note_labels == {
+        "project":         "Project:",
+        "client":          "Client:",
+        "operator":        "User Name:",
+        "sensor_location": "Seis Loc:",
+    }
+
+
+def test_user_notes_operator_renamed_labels_still_populate():
+    """If the operator renames the labels in BW's UI (e.g. "Seis Loc:"
+    → "Building:"), the values STILL populate the canonical slots by
+    position — and the operator's labels are preserved in
+    `user_note_labels` for terra-view to display."""
+    r = parse_report(_wrap_user_notes(
+        "Building : Main Office",
+        "Project Manager : Brian",
+        "Inspector : Claude",
+        "Site Address : 123 Main St",
+    ))
+    assert r.project         == "Main Office"
+    assert r.client          == "Brian"
+    assert r.operator        == "Claude"
+    assert r.sensor_location == "123 Main St"
+    assert r.user_note_labels == {
+        "project":         "Building",
+        "client":          "Project Manager",
+        "operator":        "Inspector",
+        "sensor_location": "Site Address",
+    }
+
+
+def test_user_notes_with_histogram_label_spelling():
+    """Histogram exports use 'Seis. Location:' (with period and colon)
+    instead of 'Seis Loc:'.  Position-based parsing handles both."""
+    r = parse_report(_wrap_user_notes(
+        "Project: : Plum Cont.- Rainbow Run",
+        "Client: : Plum Contracting In.c",
+        "User Name: : Terra-Mechanics Inc.",
+        "Seis. Location: : Loc #1 - 2652 Hepner",
+    ))
+    assert r.project == "Plum Cont.- Rainbow Run"
+    assert r.client == "Plum Contracting In.c"
+    assert r.operator == "Terra-Mechanics Inc."
+    assert r.sensor_location == "Loc #1 - 2652 Hepner"
+    # And the histogram's specific label spelling is preserved
+    assert r.user_note_labels["sensor_location"] == "Seis. Location:"
+
+
+def test_user_notes_outside_block_are_ignored():
+    """Lines that look like user-notes but appear OUTSIDE the
+    Units→Geo Range range don't get assigned to user-note slots."""
+    # No Units anchor — these lines shouldn't populate user-note slots
+    text = (
+        '"Serial Number : BE11529"\n'
+        '"Project: : SHOULD NOT POPULATE"\n'
+    )
     r = parse_report(text)
-    assert getattr(r, expected_field) == expected_value, (label, expected_field)
+    assert r.serial == "BE11529"
+    assert r.project is None
 
 
-def test_label_normalisation_is_case_insensitive():
-    """Lowercase / uppercase / mixed-case labels all hit the same slot."""
-    for label in ("seis loc", "SEIS LOC", "SeIs LoC"):
-        text = '"{} : Catbed"\n'.format(label)
-        r = parse_report(text)
-        assert r.sensor_location == "Catbed", label
+def test_user_notes_partial_block_only_fills_present_slots():
+    """If BW writes fewer than 4 user-notes (e.g. operator disabled
+    Extended Notes mid-block), only the present positions populate;
+    later slots stay None."""
+    r = parse_report(_wrap_user_notes(
+        "Project: : Just-a-project",
+        "Client: : Just-a-client",
+    ))
+    assert r.project == "Just-a-project"
+    assert r.client  == "Just-a-client"
+    assert r.operator is None
+    assert r.sensor_location is None
 
 
-def test_label_normalisation_collapses_extra_whitespace():
-    """Internal whitespace runs in labels are collapsed before lookup."""
-    text = '"Seis  Loc : Catbed"\n'   # two spaces between "Seis" and "Loc"
-    r = parse_report(text)
-    assert r.sensor_location == "Catbed"
+def test_user_notes_extra_lines_beyond_four_are_dropped():
+    """If somehow more than 4 lines appear in the user-notes block
+    (e.g. BW adds an Extended Notes line), only the first 4 are
+    captured — slots 5+ have nowhere to go."""
+    r = parse_report(_wrap_user_notes(
+        "L1 : v1",
+        "L2 : v2",
+        "L3 : v3",
+        "L4 : v4",
+        "L5 : v5",   # ignored — no fifth slot
+    ))
+    assert r.project         == "v1"
+    assert r.client          == "v2"
+    assert r.operator        == "v3"
+    assert r.sensor_location == "v4"
+    # 5th label not captured
+    assert "L5" not in r.user_note_labels.values()
 
 
 def test_real_histogram_fixture_populates_sensor_location():
-    """End-to-end: the histogram fixture from example-events/histogram/
-    uses 'Seis. Location' (with period) and must successfully populate
-    sensor_location after the label-normalisation fix."""
+    """End-to-end: the histogram fixture uses 'Seis. Location:' — must
+    successfully populate sensor_location via position-based parsing."""
     fixture_dir = (
         Path(__file__).parent.parent / "example-events" / "histogram"
     )
@@ -316,10 +398,9 @@ def test_real_histogram_fixture_populates_sensor_location():
         pytest.skip("no histogram TXT in fixture dir")
 
     r = parse_report_file(txt)
-    # The histogram TXTs verified to use "Seis. Location" — should now
-    # populate sensor_location instead of being silently dropped.
     assert r.sensor_location is not None
     assert len(r.sensor_location) > 0
+    assert r.user_note_labels.get("sensor_location") is not None
     # Sanity: other shared fields still parse correctly
     assert r.serial is not None
     assert r.serial.startswith("BE")