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

src/song-274/palette.ts

@@ -0,0 +1,66 @@
+/**
+ * Every colour the 274 transect video draws, named by what it is for rather than
+ * what it looks like. Change values here, or override `palette` in the Studio's
+ * input props, without touching the drawing code.
+ *
+ * The dark palette is the video's own, grown from the dark palette of
+ * `song-274/route/274-transect.html`; comments name the page's CSS variable each
+ * token corresponds to. The light palette is the page's, extended with the
+ * tokens the video added (shrine, railway).
+ */
+export type TransectPalette = {
+  /** Frame background. */
+  background: string;
+  /** The whole road, before the song has covered it. */
+  roadAhead: string;
+  /** The road the song has covered so far. */
+  roadCovered: string;
+  /** Node fill per feature family, as on the page's legend. */
+  bridge: string;
+  tunnel: string;
+  shed: string;
+  watercourse: string;
+  place: string;
+  /** Stations and every 信号場, whatever their roster kind. */
+  railway: string;
+  shrine: string;
+  other: string;
+};
+
+export type FeatureFamily =
+  | "bridge"
+  | "tunnel"
+  | "shed"
+  | "watercourse"
+  | "place"
+  | "railway"
+  | "shrine"
+  | "other";
+
+export const darkPalette: TransectPalette = {
+  background: "#28323d", // --paper
+  roadAhead: "#49565f", // --rule
+  roadCovered: "#9dcde4", // --road
+  bridge: "#79d1c5", // --bridge
+  tunnel: "#b6c5c1", // --tunnel
+  shed: "#eba9eb", // --shed
+  watercourse: "#5FB6D6", // --water
+  place: "#129252", // --place
+  railway: "#129252", // --railway
+  shrine: "#b32c2c", // --shrine
+  other: "#ff9900", // --util
+};
+
+export const lightPalette: TransectPalette = {
+  background: "#E7ECEE",
+  roadAhead: "#C0CCD2",
+  roadCovered: "#2E4A5A",
+  bridge: "#116B62",
+  tunnel: "#3A4650",
+  shed: "#7A5AA6",
+  watercourse: "#2F7E9E",
+  place: "#8A6D1F",
+  railway: "#1F6E43",
+  shrine: "#9B2C2C",
+  other: "#5B6B75",
+};