fix(highway): apply 3D fret-spacing live instead of reloading (#561) (#562)

window.h3dSetFretSpacing was the only 3D-highway setting that applied via
location.reload(). The SPA boots with #home as the active screen and has
no restore-last-screen mechanism, so the reload ejected the user from
Settings onto the home screen.

Apply it live like every other 3D-highway setting: rebind the module-scope
_h3dFretUniform flag (so panels mounted later this session pick up the new
mode), recompute the two fretX-derived scalars baked at init
(_fretLabelScaleRefW, FRET_WIDTH_MID), and broadcast a 'fretSpacing' change
over the existing _bgEmitChange pub-sub so every mounted panel rebuilds its
board via buildBoard(). Per-frame note geometry already reads fretX live.

Settings copy updated (no longer reloads) and tests/js pin the no-reload /
live-rebuild behavior.

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

tests/js/highway_3d_fret_spacing.test.js

@@ -42,3 +42,33 @@ test('h3dSetFretSpacing validates the mode against the two supported values', ()
         'h3dSetFretSpacing must coerce mode to a supported value before persisting',
     );
 });
+
+test('h3dSetFretSpacing applies the change live, not via a page reload', () => {
+    // A reload reboots the SPA to the home screen, ejecting the user from
+    // Settings. The setter must instead rebind the spacing flag and broadcast
+    // a 'fretSpacing' change so mounted panels rebuild in place — same path as
+    // every other 3D-highway setting. Reintroducing location.reload() here is
+    // the regression this guards against.
+    const src = fs.readFileSync(SCREEN_JS, 'utf8');
+    const setter = src.match(/window\.h3dSetFretSpacing\s*=\s*mode\s*=>\s*\{[\s\S]*?\n    \};/);
+    assert.ok(setter, 'h3dSetFretSpacing assignment must be present');
+    assert.doesNotMatch(
+        setter[0],
+        /location\.reload/,
+        'h3dSetFretSpacing must not reload the page (it ejects the user from Settings)',
+    );
+    assert.match(
+        setter[0],
+        /_bgEmitChange\(\s*'fretSpacing'\s*\)/,
+        'h3dSetFretSpacing must broadcast a live fretSpacing change',
+    );
+});
+
+test('the fretSpacing change rebuilds a mounted board live', () => {
+    const src = fs.readFileSync(SCREEN_JS, 'utf8');
+    assert.match(
+        src,
+        /changedKey\s*===\s*'fretSpacing'[\s\S]*?if\s*\(fretG\)\s*buildBoard\(\)/,
+        'the panel bg listener must rebuild the board when fretSpacing changes',
+    );
+});