fix(report_pdf): histogram geo channels share nice-quantized y-axis

Two related visual bugs on histogram PDFs:

1. Per-channel auto-scale meant Tran/Vert/Long had different y-axes
   (e.g. 0-0.015, 0-0.025, 0-0.020) — bars looked taller on the
   channel that happened to be quietest.  Not directly comparable.

2. Footer "Amplitude Geo: X in/s/div" was just amax/5 of the FIRST
   geo channel with data, with no LSB quantization — producing
   nonsense like 0.003 in/s/div when the geophone LSB is 0.005.

Fix: compute a single shared geo y-axis range from max(Tran,Vert,Long),
quantize the per-division step to BW's 1-2-5 sequence rounded to the
0.005 LSB (0.005, 0.01, 0.025, 0.05, 0.1, 0.25, ...), apply the same
ylim + ticks to all three geo subplots, and use that same step for the
footer label.  MicL stays on its own auto-scale (different units).

Verified across edge cases including the reported event
(geo max 0.025 → 0.005/div, top 0.025), small PVS events, and large
blast amplitudes.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

sfm/report_pdf.py

@@ -796,11 +796,31 @@ def _draw_waveform_subplot(fig, gridspec_cell, rd: ReportData) -> None:
     )
 
 
+def _nice_geo_step(amax: float) -> float:
+    """Pick a "nice" per-division step for the geo y-axis.
+
+    Geo LSB is 0.005 in/s — sub-LSB steps like 0.003/div are nonsense.
+    Quantize to the BW-style 1-2-5 sequence (0.005, 0.01, 0.025, 0.05,
+    …) and return the smallest step where 5 divisions >= amax, so the
+    top of the chart lands on a tick.
+    """
+    if amax <= 0:
+        return 0.005
+    for step in (0.005, 0.01, 0.025, 0.05, 0.1, 0.25, 0.5, 1.0, 2.5, 5.0, 10.0):
+        if step * 5 >= amax:
+            return step
+    return 10.0
+
+
 def _draw_histogram_subplot(fig, gridspec_cell, rd: ReportData) -> None:
     """4-channel stacked histogram bar chart — per-interval peaks.
 
     X-axis labeled with the actual times from rd.histogram_interval_times
     when available; otherwise interval index.
+
+    The three geo channels share a single y-axis scale (a BW-style nice
+    multiple of the 0.005 in/s LSB) so bar heights are directly
+    comparable across channels.  MicL has its own auto-scale.
     """
     inner = gridspec_cell.subgridspec(4, 1, hspace=0.0)
     order = ["MicL", "Long", "Vert", "Tran"]
@@ -809,6 +829,16 @@ def _draw_histogram_subplot(fig, gridspec_cell, rd: ReportData) -> None:
     # X-axis: use absolute time labels if we have them, else interval index
     have_times = bool(rd.histogram_interval_times)
 
+    # Shared geo scale: max across Tran/Vert/Long, quantized to a nice
+    # tick step.  Used for ylim + the footer "Amplitude Geo: X in/s/div".
+    geo_amax = 0.0
+    for gch in ("Tran", "Vert", "Long"):
+        gv = rd.channels.get(gch) or []
+        if gv:
+            geo_amax = max(geo_amax, max(abs(x) for x in gv if x is not None))
+    geo_step = _nice_geo_step(geo_amax)
+    geo_top  = geo_step * 5  # 5 divisions — top tick lands at this value
+
     for i, ch in enumerate(order):
         ax = fig.add_subplot(inner[i])
         values = rd.channels.get(ch) or []
@@ -821,6 +851,10 @@ def _draw_histogram_subplot(fig, gridspec_cell, rd: ReportData) -> None:
             xs = np.arange(len(abs_vals))
             color = _channel_axis_color(ch)
             ax.bar(xs, abs_vals, color=color, width=0.85, linewidth=0)
+            if ch in ("Tran", "Vert", "Long"):
+                ax.set_ylim(0, geo_top)
+                ax.set_yticks([j * geo_step for j in range(6)])
+            else:
                 amax = max(abs_vals, default=0)
                 if amax > 0:
                     ax.set_ylim(0, amax * 1.10)
@@ -846,15 +880,11 @@ def _draw_histogram_subplot(fig, gridspec_cell, rd: ReportData) -> None:
             ax.tick_params(axis="x", labelsize=7)
         ax.tick_params(axis="y", labelsize=6)
 
-    # Footer scale info — histograms use minute/div
+    # Footer scale info — histograms use minute/div.  Reuses the shared
+    # geo_step computed above so the label matches the actual y-axis
+    # tick spacing on every subplot.
     interval_str = rd.histogram_interval_size or "—"
-    geo_amp_div = "—"
-    for ch in ("Tran", "Vert", "Long"):
-        v = rd.channels.get(ch) or []
-        if v:
-            amax = max(abs(x) for x in v)
-            geo_amp_div = f"{amax / 5:.3f}"
-            break
+    geo_amp_div = f"{geo_step:.3f}"
     fig.text(
         0.11, 0.030,
         f"Time {interval_str} /div   Amplitude Geo: {geo_amp_div} in/s/div   Mic: 0.001 psi(L)/div",