feat(274): transect composition synced to the lyric

Song274Transect draws Route 274 in its true shape (the "Ground" view of
song-274/route/274-transect.html) and plays it along with the song.

- data.ts: reads song-274/route/ at load time (lyrics index, roster, route and
  the optional east/west stubs); nothing is copied into the code. Only sung
  nodes are shown; districts and 東に続く light nothing.
- progress.ts: a "covered so far" paint over the dimmed road. Its head is pinned
  to each sung object's km as that line starts and glides between pins on a
  monotone cubic, so it never runs backwards. After 東に続く it runs out along
  the east stub; the west stub is always painted.
- camera.ts: fixed close zoom during each narration part (lines under 20 s
  apart), following a Gaussian-smoothed head; pull back to frame the covered
  road plus some road ahead during breaks, and to the full map after the lyric.
- nodeAnimation.ts: lightUp / switchOff as reusable functions over a NodeLook,
  with long lines switching off after MAX_LIT_S.
- palette.ts: every colour as a named token (dark default, light alongside),
  including shrine and railway families.

remotion.config.ts sets song-274/ as the public dir so staticFile() reaches the
song's audio/ and route/ in place.

Verified: eslint + tsc clean; stills checked across parts, breaks and the ending.
Render with --muted and add audio with ffmpeg: Remotion's own AAC lands 42.75 ms
late.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PLt25nCVMsoroKALYUBu6u

remotion.config.ts

@@ -12,3 +12,6 @@ Config.setRspack(true);
 Config.setVideoImageFormat("jpeg");
 Config.setOverwriteOutput(true);
 Config.overrideBundlerConfig(enableTailwind);
+// Song branch: staticFile() paths resolve inside the song folder, so compositions
+// read audio/, fonts/ and route/ where they already live instead of a copy.
+Config.setPublicDir("song-274");