feat: headless PNG render, mLnL annotations, and per-frame sched offset

Commit the accumulated working-tree changes as one snapshot.

- Headless render: `--render OUT.png INPUT.wav` draws the spectrogram
  (full window, or `--pane` for the spectrogram pane only) to a PNG
  with no visible window. Options: `--annotations`/`--no-annotations`,
  `--annotation-opacity`, `--width`/`--height`.
- mLnL annotations: parse the optional `mLnL` RIFF chunk (schema v2)
  and render tx_frame/assertion/control overlays, a timeline lane, and
  a waveform-scope echo, with hover tooltips on the spectrogram,
  timeline, and scope.
- sched_offset_ms: parse the per-frame intent->air latency and surface
  it in the hover tooltips (boxes stay air-anchored upstream).
- Supporting: build wiring (rspektrum.make), shared types/headers,
  web-build and capture-script tweaks, and removal of the old
  synchrosqueezing LaTeX doc.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

shot_input.sh

@@ -25,7 +25,7 @@
 #   a selection box needs a >5 px drag or it reverts to "no selection".
 set -u
 OUT="${1:-/tmp/rspek_shot.png}"; shift || true
-REPO=/home/tyler/ham/Audio-Experiments/rspektrum
+REPO="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
 DISP=:99
 WAV="${RSPEK_WAV:-/tmp/mlink_bulk_drop.wav}"