feat(highway_3d): fret wires flash on a confirmed hit

The fret wires were static scenery: gold inside the anchor lane, grey
outside, and nothing tied them to what the player was actually doing.

Give them a job. Widen the lane/neck contrast so the wires around the
active lane read as a focus cue, and flash the wires bracketing a note
when a scorer confirms it. A fretted note lights the wire behind it and
the wire it is pressed against; a chord lights only the outermost wires
of its shape, so it reads as one bracketed block rather than a picket
fence; an open string has no fret of its own and its gem is drawn as a
slab spanning the lane, so it lights the lane's edge wires instead.

Gated on the provider verdict, never the proximity heuristic -- the
latter only means "near the strike line", so it would flash on every
passing note whether or not it was played. With no scorer attached the
neck behaves exactly as before.

Emissive (and emissiveIntensity) carry the flash, not albedo: these are
MeshStandard materials in a scene with no envMap, so raising albedo
alone barely brightens them.

Every value is a named constant -- see FRET_WIRE_* -- because the look
is a taste call that wants tuning by eye, not a derivation.

Signed-off-by: Kris Anderson <topkoa@gmail.com>

CHANGELOG.md

@@ -44,6 +44,13 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   data-driven bar (avg ≥ 75%) — into the career state; abandoned sets never
   log (no fail state: the gig you finished is the gig you played). Passports
   carry their gig log; instruments their gig count.
+- **3D Highway: fret wires flash on a confirmed hit** — when a scorer (note_detect)
+  confirms a note through the `getNoteState` provider (feedBack#254), the fret wires
+  bracketing it light up. A fretted note lights the wire behind it and the wire it's
+  pressed against; a chord lights only the outermost wires of its shape; an open
+  string lights the anchor lane's edge wires (its gem is drawn as a slab spanning the
+  lane, so those are the wires it sits between). With no scorer attached, nothing
+  changes.
 
 ### Changed
 - **Folder library renders only the songs on screen** (#965) — a song list used to
@@ -54,6 +61,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   tree — which is how the song-preview menu check ended up eating ~50% of the
   renderer and dropping the app to 2.7 fps. Lists longer than 200 songs are now
   windowed (25–31 rows in the DOM instead of 50,000); shorter lists are unchanged.
+- **3D Highway: fret wires read as a focus cue** — the contrast between the active
+  anchor lane and the rest of the neck is widened (the lane's wires brighter, the rest
+  dimmer), and the wires themselves are slightly thicker.
 - **The full mix is a stem** (#933) — core no longer depends on `original_audio:`, a
   top-level manifest key this repo invented (#583) that the feedpak spec never had.
   The format already carried the pre-separation mixdown as a stem; feedpak 1.15.0