fix(web): rail polish — waveform fits, mirror is smooth

- WaveformCanvas: a ResizeObserver keeps the drawing buffer matched to the
  element size (was measured once at mount), so the trace is centered and peaks
  never clip; taller 100px window; clamp to ±45% height.
- useTelemetry: signed oscilloscope signal (idle wave at rest, decaying ring on
  impact, bounded ±0.95) instead of a one-sided magnitude that overshot the box.
- useMatrixMirror: poll 8 → 15 fps. The board renders at ~12 fps and a matrix_get
  round-trip is only ~15 ms, so 8 fps under-sampled (stutter); 15 fps oversamples
  cleanly, well under the ~68 fps ceiling.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

src/components/cockpit/CockpitRail.tsx

@@ -158,7 +158,7 @@ export function CockpitRail() {
         {tel.live ? (
           <WaveformCanvas wave={tel.wave} impact={tel.event === 'impact'} />
         ) : (
-          <div className="flex h-[78px] items-center justify-center rounded-[10px] border border-rail-line bg-rail-inset font-mono text-[10px] text-rail-dim3">
+          <div className="flex h-[100px] items-center justify-center rounded-[10px] border border-rail-line bg-rail-inset font-mono text-[10px] text-rail-dim3">
             adxl355_stream — awaiting board
           </div>
         )}