refactor(highway): make the highway global explicit before the module flip (R3c) (#912)

73 bare `highway.x` references -> `window.highway.x`, across app.js and 10 other files.
Provably a NO-OP today. It is the precondition for flipping highway.js to a module.

━━━ WHY THIS HAS TO LAND FIRST ━━━

highway.js is a CLASSIC script. Its top-level `const highway = createHighway()` therefore
creates a GLOBAL LEXICAL BINDING — visible as a bare name to every other classic script AND
to every ES module. 73 call sites quietly rely on that.

The moment highway.js becomes a module, that binding is gone. `const` in a module is
module-scoped, not global. Every one of those 73 sites becomes a ReferenceError, and the
flip is impossible until they say what they mean.

`window.highway = highway` is already set, to the same object, on the same line. So this is
an identity rewrite — verified in the browser below.

━━━ THE REWRITE BIT ME THREE TIMES. REGEX IS NOT ENOUGH FOR THIS. ━━━

1. A SHADOWED LOCAL. capabilities/note-detection.js does `const highway = window.highway`.
   Its 9 bare uses are LOCAL and already correct; a blind rewrite would have emitted
   `const window.highway = window.highway`. Excluded.

2. HALF-CONVERTED GUARDS — the dangerous one. Six sites read
   `typeof highway !== 'undefined' && highway && typeof highway.setTime === 'function'`.
   The regex converted the CONSEQUENT and left the TEST, which is WORSE than not touching
   them: after the flip `typeof highway` is 'undefined', so each guard is PERMANENTLY FALSE
   and the code behind it silently never runs. transport.js's was the seek->setTime sync:
   the chart clock would have quietly desynced after every seek, with nothing failing.
   All six now test window.highway.

3. TWO MORE BARE REFERENCES, found by Codex [P2] and confirmed by an AST scan: app.js:3114
   and :3176 use `highway && typeof window.highway.getSections === 'function'`. My grep
   searched for `typeof highway`, not `highway &&`. After the flip these throw, the catch
   swallows it, and the editor silently falls back to a ±4s edit window and arrangement 0.

Regex missed a shadow, a half-conversion, and two bare reads. The final check is an
AST pass that resolves scopes and reports every `highway` identifier not bound locally.
It now reports ZERO.

VERIFIED. A/B against origin/main in two browsers, 15 probes, IDENTICAL, zero page errors:
window.highway is the same object as the bare global, the whole API surface resolves, a real
song plays, the chart clock advances, getPerf().drawMs > 0 — and `seek syncs chart` passes,
which is the exact guard I nearly broke in (2).

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

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

static/js/section-practice.js

@@ -38,7 +38,7 @@ export function _sectionPracticeBarContains(el) {
 }
 
 // ── Section Practice Bar ────────────────────────────────────────────────
-// One-click looping over song section markers (highway.getSections —
+// One-click looping over song section markers (window.highway.getSections —
 // same array as 3D highway bundle.sections / "Now / Up Next").
 // Reuses setLoop() so manual A/B controls and saved loops stay canonical.
 let _sectionPracticeRanges = [];
@@ -127,7 +127,7 @@ export function _resetSectionPracticeLog() {
 }
 
 function _sectionPracticeHighway() {
-    return window.highway || (typeof highway !== 'undefined' ? highway : null);
+    return window.highway || null;
 }
 
 function _sectionPracticeDuration() {