fix(pdf): shared geo Y scale across Long/Vert/Tran (was per-trace)

The event-report waveform plot scaled each geo lane to its own peak, so a small
channel filled its lane looking as big as a large one — and the "Geo: X in/s/div"
footer only reflected whichever channel was checked first, so its div value was
wrong for the other two. Now all three geo lanes share ONE symmetric scale =
max |sample| across them (padded, 0.05 in/s floor), matching the event modal and
BW's single amp/div; the footer reflects that shared scale. Mic keeps its own psi
scale. Big events are unchanged (e.g. BE12844 stays 0.185 in/s/div).

Test-first: tests/test_report_pdf_geo_scale.py (shared scale + floor), 2 tests.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01YDXjZCr4RqT2U3QvMDhgzf

tests/test_report_pdf_geo_scale.py

@@ -0,0 +1,61 @@
+"""The event-report PDF must draw the three geo channels on ONE shared Y scale
+(max |sample| across Long/Vert/Tran, floored), not each trace auto-zoomed to its
+own peak — so relative amplitudes are honest and a small channel doesn't fill its
+lane looking as big as a large one.  Mirrors the event-modal waveform behaviour.
+"""
+import matplotlib
+matplotlib.use("Agg")
+import matplotlib.pyplot as plt
+import pytest
+
+from sfm.report_pdf import ReportData, _draw_waveform_subplot
+
+
+def _draw(channels):
+    rd = ReportData(
+        channels=channels,
+        sample_rate_sps=1024,
+        dt_ms=1000.0 / 1024,
+        t0_ms=0.0,
+    )
+    fig = plt.figure()
+    cell = fig.add_gridspec(1, 1)[0, 0]
+    _draw_waveform_subplot(fig, cell, rd)
+    by_label = {ax.get_ylabel(): ax for ax in fig.axes}
+    try:
+        yield_ = {k: by_label[k].get_ylim() for k in ("Long", "Vert", "Tran", "MicL")}
+    finally:
+        plt.close(fig)
+    return yield_
+
+
+def test_geo_traces_share_one_y_scale():
+    # Tran is the biggest geo channel (0.35); Long 0.10, Vert 0.02.
+    ylims = _draw({
+        "Long": [0.10, -0.10, 0.0],
+        "Vert": [0.02, -0.02, 0.0],
+        "Tran": [0.35, -0.35, 0.0],
+        "MicL": [0.0005, -0.0005, 0.0],
+    })
+    # Shared scale = max(0.35 * 1.10, floor 0.05) = 0.385, symmetric.
+    expected = pytest.approx(0.385, rel=1e-6)
+    for ch in ("Long", "Vert", "Tran"):
+        lo, hi = ylims[ch]
+        assert hi == expected, f"{ch} top ylim {hi} != shared 0.385"
+        assert lo == pytest.approx(-0.385, rel=1e-6), f"{ch} bottom ylim {lo}"
+    # All three geo lanes identical.
+    assert ylims["Long"] == ylims["Vert"] == ylims["Tran"]
+    # Mic keeps its own (much smaller) scale — not lumped into the geo max.
+    assert ylims["MicL"][1] < 0.01
+
+
+def test_geo_shared_scale_has_floor():
+    # A tiny event (all geo well under the floor) clamps to the 0.05 floor.
+    ylims = _draw({
+        "Long": [0.008, -0.008, 0.0],
+        "Vert": [0.006, -0.006, 0.0],
+        "Tran": [0.010, -0.010, 0.0],
+        "MicL": [0.0001, -0.0001, 0.0],
+    })
+    for ch in ("Long", "Vert", "Tran"):
+        assert ylims[ch][1] == pytest.approx(0.05, rel=1e-6), f"{ch} not floored"