feat(highway_3d): cap the fret-wire flash at one outer pair

Fast passages overlap their decay tails: consecutive notes on nearby
frets left three, four, five wires glowing at once — the picket fence
the chord rule was written to avoid, arriving through time instead of
through a shape.

The apply pass now decays every wire's glow state as before, but flashes
only the outermost pair of the lit span (or the single wire when only
one is above threshold). Interior wires keep decaying invisibly — the
base tier loop re-seeds their materials each frame — so the bracket
tightens naturally as the outer tails expire, and a hit inside the
current span widens nothing.

Net effect: at most two wires are ever lit, and everything currently
glowing reads as one bracket, exactly like a chord.

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

CHANGELOG.md

@@ -49,8 +49,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   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.
+  lane, so those are the wires it sits between). 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.
 
 ### Changed
 - **`GET /api/song/{f}?stems=1`** (new, opt-in) — returns the pack's playable stem