fix(forward): pair BW ACH ASCII reports using the _ASCII.TXT convention (v1.5.4)

Blastware's official Auto Call Home server writes per-event ASCII
reports as <stem>_<ext>_ASCII.TXT (e.g. N844L20G_630H_ASCII.TXT),
not <binary>.TXT (e.g. N844L20G.630H.TXT).  Versions v1.5.0–v1.5.3
only looked for the latter and silently shipped every binary alone,
so the SFM database lost the per-event Peak Acceleration / Peak
Displacement / ZC Freq / Time of Peak / Peak Vector Sum + time /
sensor self-check fields on every forwarded event.

Fix: pair-finding logic now tries the ACH-convention filename first
and falls back to <binary>.TXT for compatibility with operator-saved
manual exports and existing test fixtures.

  ach_report_name("M529LK44.AB0")    → "M529LK44_AB0_ASCII.TXT"
  legacy_report_name("M529LK44.AB0") → "M529LK44.AB0.TXT"

When both files exist (operator manually saved + ACH auto-exported),
ACH wins because that's the canonical name on modern BW deployments.
Both candidates checked case-insensitively against the cached
directory listing — no extra stat() calls.

6 new unit tests cover the new pairing logic, helper-function
correctness, and the precedence rule.  Total now 31 tests, all green.

Field-deploy note: re-running v1.5.4 on a folder where v1.5.0–v1.5.3
already ran will NOT re-forward historical events — the
sfm_forwarded.json state file remembers them by sha256.  To re-forward
historical events to populate SFM with the now-correctly-paired
reports, delete the state file before starting v1.5.4.

series3_watcher.py

@@ -247,7 +247,7 @@ def scan_latest(
 
 
 # --- API heartbeat / SFM telemetry helpers ---
-VERSION = "1.5.3"
+VERSION = "1.5.4"
 
 
 def _read_log_tail(log_file: str, n: int = 25) -> Optional[list]: