refactor(highway): carve the constants into static/js/highway-constants.js (R3c) (#914)

29 constants, 190 lines. highway.js 4,267 -> 4,158. The first real slice, and the one that
every later one imports.

━━━ WHY ONLY THE CONSTANTS MAY LIVE AT MODULE SCOPE ━━━

createHighway() is a FACTORY, not a singleton. The constitution publishes
window.createHighway precisely so a plugin can build a SECOND highway for its own panel, and
highway.js already says so at the top of the closure:

    // R3c: per-instance mutable state in one object, so extracted renderer/ws
    // modules can close over it as a factory arg without cross-panel sharing.

So hwState — all 79 mutable properties — must NEVER become a module-level singleton: two
highways would silently share it, and one panel would drive the other's clock, scale and
colour tables. Extracted functions will take it as an ARGUMENT.

That is the OPPOSITE of the app.js carve, where a single state container (player-state.js,
library-state.js) was exactly right, because there is exactly one app. Same epic, same
language, opposite answer — because one is a singleton and the other is a factory.

These 29 are pure literals: numbers, strings and colour tables, never reassigned, never
mutated. Sharing them across instances is not merely safe, it is what you want — one copy of
the shimmer LUT bounds and the string palettes rather than one per panel. Anything with a
runtime dependency (document, window, performance, localStorage) stays in the factory;
checked, and none of these has one.

ESLint now knows static/highway.js is a module. It could not have known before this commit:
the flip (#913) changed the SCRIPT TAG, but the file had no import/export yet, so it still
parsed as a script and lint stayed green. The first `import` is what makes the config wrong.

TESTS. Four source-shape harnesses asserted `const _AUTO_SCALE_MIN = …` etc. lived in
highway.js. They now read highway.js AND every static/js/highway-*.js — deliberately, rather
than being re-pinned at whichever file currently holds a constant. Re-pinning just breaks
again on the next carve, and a source-shape assertion that silently stops finding its target
is indistinguishable from one that passes. Bite-tested: renaming two constants away fails
them.

VERIFIED. A/B against origin/main: 15 probes IDENTICAL, zero page errors. AND THE PERF GATE
PASSES AT 1.97ms against its 12ms budget — which is the point of having built it (#910)
first: these constants moved from closure scope to module scope, and V8 does not treat those
identically. It does here. Now I know rather than hope.

node 1045, pytest 2416, ESLint 0, Codex 0.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/js/highway_monotonic_clock.test.js

@@ -60,7 +60,7 @@ function buildClockSandbox(perfNowImpl) {
         performance: { now: perfNowImpl },
     };
     vm.createContext(sandbox);
-    const src = fs.readFileSync(HIGHWAY_JS, 'utf8');
+    const src = highwaySources();
     const setTimeBody = extractBlock(src, 'setTime(t) {');
     const getTimeBody = extractBlock(src, 'getTime() {');
     // Strip trailing comma if present (object-literal method declarations).
@@ -72,8 +72,25 @@ function buildClockSandbox(perfNowImpl) {
     return sandbox;
 }
 
+
+// R3c: highway.js is being carved into modules, so its source is no longer ONE file. Read the
+// whole set. Re-pinning these assertions at whichever file currently holds a constant just
+// means they break again on the next carve — and worse, a source-shape assertion that silently
+// stops finding its target is indistinguishable from one that passes.
+function highwaySources() {
+    const root = path.join(__dirname, '..', '..');
+    const jsDir = path.join(root, 'static', 'js');
+    const parts = [fs.readFileSync(path.join(root, 'static', 'highway.js'), 'utf8')];
+    for (const f of fs.readdirSync(jsDir).sort()) {
+        if (f.startsWith('highway-') && f.endsWith('.js')) {
+            parts.push(fs.readFileSync(path.join(jsDir, f), 'utf8'));
+        }
+    }
+    return parts.join('\n');
+}
+
 test('highway declares chart anchor + stall-detect + rate state', () => {
-    const src = fs.readFileSync(HIGHWAY_JS, 'utf8');
+    const src = highwaySources();
     // Both anchor fields use NaN sentinels — _chartAnchorAudioT in
     // particular MUST start as NaN, not 0, otherwise setTime(0) on the
     // very first 60 Hz tick fails the `t !== _chartAnchorAudioT` check
@@ -82,11 +99,11 @@ test('highway declares chart anchor + stall-detect + rate state', () => {
     assert.match(src, /hwState\._chartAnchorPerfNow\s*=\s*NaN/, 'missing _chartAnchorPerfNow (NaN sentinel)');
     assert.match(src, /hwState\._chartLastAdvanceAt\s*=\s*0/, 'missing _chartLastAdvanceAt (pause detection)');
     assert.match(src, /hwState\._chartObservedRate\s*=\s*1/, 'missing _chartObservedRate (playback rate awareness)');
-    assert.match(src, /const\s+_CHART_MAX_INTERP_MS\s*=\s*100/, 'missing _CHART_MAX_INTERP_MS cap');
+    assert.match(src, /(?:export\s+)?const\s+_CHART_MAX_INTERP_MS\s*=\s*100/, 'missing _CHART_MAX_INTERP_MS cap');
 });
 
 test('getTime scales interpolation by _chartObservedRate (speed-slider safe)', () => {
-    const src = fs.readFileSync(HIGHWAY_JS, 'utf8');
+    const src = highwaySources();
     const m = src.match(/getTime\(\)\s*\{[\s\S]+?\n\s*\},/);
     assert.ok(m, 'getTime() body not found');
     const slice = m[0];
@@ -98,7 +115,7 @@ test('getTime scales interpolation by _chartObservedRate (speed-slider safe)', (
 });
 
 test('setTime re-anchors and updates _chartLastAdvanceAt only when t actually changes', () => {
-    const src = fs.readFileSync(HIGHWAY_JS, 'utf8');
+    const src = highwaySources();
     // Repeated setTime calls with the same value must not refresh the
     // anchor (else interpolation stutters); they also must not refresh
     // _chartLastAdvanceAt (else getTime would never detect a stalled
@@ -115,7 +132,7 @@ test('setTime re-anchors and updates _chartLastAdvanceAt only when t actually ch
 });
 
 test('getTime falls back to chartTime when audio has stalled (paused)', () => {
-    const src = fs.readFileSync(HIGHWAY_JS, 'utf8');
+    const src = highwaySources();
     // Find the actual getTime body. Match the whole brace-balanced
     // method (using a generous greedy slice to ensure we capture both
     // the stall check and the interpolation expression below it).
@@ -139,7 +156,7 @@ test('getTime falls back to chartTime when audio has stalled (paused)', () => {
 });
 
 test('api.stop() clears the chart anchor state so re-init starts fresh', () => {
-    const src = fs.readFileSync(HIGHWAY_JS, 'utf8');
+    const src = highwaySources();
     // Use the brace-balanced extractor so the assertions are scoped to
     // the actual stop() body — a fixed-size slice would falsely match
     // resets that landed in an adjacent method.