viewers: render pre-trigger samples (time_axis is metadata, not an array)

The /db/events/{id}/waveform.json endpoint returns `time_axis` as a
metadata object — {sample_rate, pretrig_samples, t0_ms, dt_ms,
n_samples, total_samples, rectime_seconds} — not a per-sample times
array.  Both viewers (sfm_webapp.html sidecar modal + event_browser.html)
were treating it as an array, silently falling back to a derived path
that ignored pretrig entirely and started the time axis at 0.

Symptom: trigger line drawn at the very left edge of every chart, no
visible "leading up to the event" samples even though they're in the
decoded data.

Fix: read time_axis.t0_ms (negative when pretrig samples exist),
time_axis.dt_ms, build per-sample times as `t0_ms + i * dt_ms`.  Trigger
line lands at sample where t crosses 0; pretrig samples render at
negative t to the left of it.

Confirmed on a K558 event with 208 pretrig samples + 2 sec rectime at
1024 sps — time axis now spans -203 ms to +2046 ms, trigger line at
~9% from the left edge as expected.

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

sfm/event_browser.html

@@ -522,8 +522,13 @@ function renderWaveform(data) {
   charts = {};
 
   const channels = data.channels || {};
-  const timeAxis = data.time_axis || null;  // ms relative to trigger
+  // time_axis is METADATA from sfm.plot.v1 — sample_rate, pretrig_samples,
-  const triggerMs = data.trigger_ms ?? 0;
+  // t0_ms (first-sample time relative to trigger; negative when pretrig
+  // exists), dt_ms.  Trigger is at t=0 by convention.
+  const ta    = data.time_axis || {};
+  const sr    = ta.sample_rate || 1024;
+  const dtMs  = ta.dt_ms || (1000.0 / sr);
+  const t0Ms  = ta.t0_ms != null ? ta.t0_ms : 0;
   const isPrintMode = document.body.classList.contains('print-view');
 
   // Which channels actually have data → determines which one renders the
@@ -578,14 +583,8 @@ function renderWaveform(data) {
     wrap.appendChild(canvasWrap);
     chartsDiv.appendChild(wrap);
 
-    // Build time labels — use server-provided time_axis if present, else derive from sample_rate
+    // Per-sample time in ms relative to trigger.  Negative for pre-trigger samples.
-    let times;
+    const times = values.map((_, i) => t0Ms + i * dtMs);
-    if (timeAxis && timeAxis.length === values.length) {
-      times = timeAxis;
-    } else {
-      const sr = data.sample_rate || 1024;
-      times = values.map((_, i) => (i / sr * 1000 - triggerMs));
-    }
 
     // Downsample for rendering
     const MAX_POINTS = 4000;

sfm/sfm_webapp.html

@@ -2572,8 +2572,14 @@ function _renderScWaveform(data) {
   _destroyScCharts();
 
   const channels = data.channels || {};
-  const timeAxis = data.time_axis || null;
+  // time_axis is METADATA, not an array — it carries sample_rate,
-  const triggerMs = data.trigger_ms ?? 0;
+  // pretrig_samples, t0_ms (first-sample time relative to trigger,
+  // negative when pretrig samples exist), and dt_ms.  Trigger is at
+  // t=0 by convention.
+  const ta       = data.time_axis || {};
+  const sr       = ta.sample_rate || 1024;
+  const dtMs     = ta.dt_ms || (1000.0 / sr);
+  const t0Ms     = ta.t0_ms != null ? ta.t0_ms : 0;
 
   // Which channels have data — determines which one renders the shared bottom axis.
   const withData = _SC_CHANNEL_ORDER.filter(ch =>
@@ -2612,14 +2618,8 @@ function _renderScWaveform(data) {
     wrap.appendChild(canvasWrap);
     chartsDiv.appendChild(wrap);
 
-    // Build time axis.  Prefer server-provided time_axis; else derive from sample_rate.
+    // Per-sample time in ms relative to trigger.  Negative for pre-trigger samples.
-    let times;
+    const times = values.map((_, i) => t0Ms + i * dtMs);
-    if (timeAxis && timeAxis.length === values.length) {
-      times = timeAxis;
-    } else {
-      const sr = data.sample_rate || 1024;
-      times = values.map((_, i) => (i / sr * 1000 - triggerMs));
-    }
 
     // Downsample for rendering when very long.
     const MAX = 3000;