feat(highway_3d): gem rims flash string-coloured, wire-fashion

On a confirmed hit the gem's outline now flashes in the STRING'S OWN
colour with the same intensity treatment as the fret wires — the
FRET_WIRE_HIT_INTENSITY emissive ramp, faded by the provider's alpha —
instead of the fixed spring-green mHitBright rim. Just the rims: the
lateral face fill keeps its existing green, and the sustain trail is
untouched.

Mechanics mirror the wires' pattern. mRimFlash[s] is one material per
string (created with the other per-string materials, palette-retint
aware, fog-exempt, disposed in teardown); drawNote() assigns it as the
outline on a good verdict and records the verdict alpha into a
per-frame per-string max (_rimFlashIn); the flash pass applies the
intensity ramp once per string. Shared-per-string is the same
compromise mGlow already makes — two same-string gems flashing in
different phases share the brighter alpha.

No decay tail of our own, deliberately: the material is only assigned
while the provider confirms the note, and the provider's alpha already
fades. When it goes silent the outline reverts, so idle intensity never
shows.

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

CHANGELOG.md

@@ -55,7 +55,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   survives only as the fallback on anchor-less charts). At most **two wires are ever lit at
   once**: when overlapping decay tails (fast passages) would light a run of wires, the
   flash collapses to the outermost pair of the lit span — one bracket, never a picket
-  fence. With no scorer attached, nothing changes.
+  fence. The gem's rim joins in: on a confirmed hit the outline flashes in the
+  string's own colour with the same intensity treatment as the wires, fading with the
+  scorer's alpha. With no scorer attached, nothing changes.
 
 ### Changed
 - **`GET /api/song/{f}?stems=1`** (new, opt-in) — returns the pack's playable stem