Metabolism: song as substrate, ecology grown from the track

A new top-level section building the metabolism concept: the track
supplies the food field, event impulses and per-section laws; the seed
picks the species (a rule set, not a parameter vector). Same song + seed
=> bit-identical frames; different songs => different worlds.

- analysis: single FFT pass -> per-frame feature table; score (onsets
  folded into world geometry via voice ring + pitch-class sector), wells
  (food placed by the harmony), graph (band correlations -> interaction
  matrix), laws (per-section regimes)
- sim: typed-array world, 3 castes (builder/orbit, forager/commute,
  spark/orbit-wide) with causality rings, persistence trails,
  consequence field; fully deterministic
- lens: software framebuffer renderer (PNG in node, ImageData in
  browser) + event-reactive camera (cuts on activity change, alternating
  shot sizes, drift-and-push)
- metrics: coupling x residual fitness (not per-frame descriptors) plus
  a grown-structure fingerprint; report + strip tools; 15 tests green
- DESIGN.md captures the concept, invariants and build order

3 reference test signals (drone/runner/dusk) grow measurably distinct
worlds while seeds of one song stay in the same biome.

metabolism/tools/strip.js

@@ -0,0 +1,73 @@
+// Render one filmstrip per song: N frames evenly across the track, tiled.
+//
+//   node tools/strip.js --songs drone,runner,dusk
+//   node tools/strip.js --songs drone --cols 6 --rows 2 --w 480 --h 270
+//
+// Frames come from the same Run as a full render, and the sim is advanced
+// through every frame in between, so a strip cannot show a world that the
+// video would not have.
+
+import { writeFileSync, mkdirSync } from 'node:fs';
+import { resolve, dirname } from 'node:path';
+import { fileURLToPath } from 'node:url';
+import { loadSong } from './load.js';
+import { Run } from '../src/film.js';
+import { encodePNG, tile } from '../src/render/png.js';
+
+const HERE = dirname(fileURLToPath(import.meta.url));
+const ROOT = resolve(HERE, '..');
+const BANK = resolve(ROOT, '../flow-state/test/songs');
+
+function args(argv) {
+    const out = {};
+    for (let i = 2; i < argv.length; i += 2) out[argv[i].replace(/^--/, '')] = argv[i + 1];
+    return out;
+}
+
+const opt = args(process.argv);
+const names = (opt.songs || 'drone,runner,dusk').split(',').map((s) => s.trim()).filter(Boolean);
+const seed = Number(opt.seed || 12345);
+const cols = Number(opt.cols || 4);
+const rows = Number(opt.rows || 3);
+const width = Number(opt.w || 480);
+const height = Number(opt.h || 270);
+const fps = 30;
+const everything = opt.combined === '1';
+const outDir = resolve(ROOT, opt.out || 'out');
+mkdirSync(outDir, { recursive: true });
+
+const count = cols * rows;
+const all = [];
+
+for (const name of names) {
+    const { song, analysis } = loadSong(resolve(BANK, `${name}.wav`), { fps, seed });
+    const run = new Run(song, { fps, width, height, seed });
+    const frames = [];
+    for (let i = 0; i < count; i++) {
+        const t = (0.10 + 0.85 * (i / Math.max(1, count - 1))) * analysis.duration;
+        frames.push(Math.min(analysis.frames - 1, Math.round(t * fps)));
+    }
+    const rendered = run.renderAt(frames);
+    const shots = frames.map((f) => rendered.get(f));
+    const zooms = shots.map((s) => `${s.cam.zoom.toFixed(1)}@${s.shot}`).join(' ');
+    const sheet = tile(shots, cols, rows, 2);
+    const file = resolve(outDir, `strip-${name}.png`);
+    writeFileSync(file, encodePNG(sheet.w, sheet.h, sheet.data));
+    const stats = run.world.stats();
+    console.log(`${file}  ${sheet.w}x${sheet.h}  ${song.name}  peak=${stats.peak}  pop=${run.world.count}  hue=${song.palette.hue.toFixed(2)}  key=${song.summary.key}`);
+    console.log(`  shots: ${zooms}`);
+    all.push({ name, song, shots, stats });
+}
+
+if (everything) {
+    // One row per song, same fractions of each track: the comparison sheet.
+    const per = 4;
+    const chosen = [];
+    for (const entry of all) {
+        for (let i = 0; i < per; i++) chosen.push(entry.shots[Math.round((i / (per - 1)) * (entry.shots.length - 1))]);
+    }
+    const sheet = tile(chosen, per, all.length, 3);
+    const file = resolve(outDir, 'strip-all.png');
+    writeFileSync(file, encodePNG(sheet.w, sheet.h, sheet.data));
+    console.log(`${file}  ${sheet.w}x${sheet.h}`);
+}