refactor(app): carve the tuning-display helpers out of app.js (R3a) (#884)

static/js/tuning-display.js (228 lines) — bodies VERBATIM. app.js 9,838 → 9,650.
A LEAF: imports nothing.

Tuning NAME resolution (Drop D / Eb Standard / raw-offset fallback), bass
detection, effective string count, and the target FREQUENCIES + note names the
tuner checks against. Pure functions over a small MIDI/note-name table; the 3
_TUNING_* tables are read nowhere else and move in.

NOT A SLICE — a node-level extract. The span 2309-2535 INTERLEAVES the functions
with the `window.*` / `window.feedBack.*` assignments that publish them, and one
of those is `window.feedBack = window.feedBack || {}` — the BUS BOOTSTRAP, not
tuning code at all. Every ExpressionStatement stays exactly where it was; only the
16 functions and 3 tables move. app.js re-exposes the imported bindings from the
same lines, so the public surface and its ordering are untouched (constitution II
names window.feedBack).

  app.js -> { plugin-loader, viz, diagnostics-export, dom, highway-colors,
              tuning-display }

HARNESSES — 4 broke, and 3 of them broke in the SAME informative way: they sliced
app.js from `function isBassArrangement(` UP TO the marker
`window.feedBack.parseRawTuningOffsets = parseRawTuningOffsets;` — an end-marker
that (correctly) stayed behind in app.js. The module is now nothing BUT the tuning
helpers, so there is no block to slice: they read it whole and strip `export ` so
the vm sandbox still evaluates it as a script.
  tuner_auto_open is SPLIT — its autoplay-gate test still reads app.js, so it keeps
  APP_JS and gains TUNING_JS. Retargeting its path wholesale (my first attempt)
  silently pointed the autoplay test at the wrong file.

VERIFIED BY DRIVING THE CONTRACT. A/B against origin/main in two browsers, through
the real window surface: displayTuningName -> "E Standard" / "Drop D" /
"Eb Standard", parseRawTuningOffsets('-2,0,0,0,0,0') -> [-2,0,0,0,0,0],
isBassArrangement, effectiveStringCount, displayTuningTargets, and
window.feedBack.displayTuningName / .songTuningContext — IDENTICAL on both, zero
console/page errors either side.

pytest 2396, node 1038/1038, ESLint 0, tailwind-fresh clean, Codex 0.

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

tests/js/tuning_display.test.js

@@ -6,7 +6,8 @@ const fs = require('node:fs');
 const path = require('node:path');
 const vm = require('node:vm');
 
-const APP_JS = path.join(__dirname, '..', '..', 'static', 'app.js');
+// The tuning-display helpers were carved out of app.js into their own module (R3a).
+const APP_JS = path.join(__dirname, '..', '..', 'static', 'js', 'tuning-display.js');
 const HIGHWAY_JS = path.join(__dirname, '..', '..', 'static', 'highway.js');
 const V3_HTML = path.join(__dirname, '..', '..', 'static', 'v3', 'index.html');