perf: remove throttled-trace residuals — program churn, per-frame rect, HUD clock

A 4x-CPU-throttled retrace (the honest weak-hardware proxy) surfaced
three residual per-frame costs; stack attribution pinned each:

- getParameters/getProgramCacheKey (~4% of main thread): every pooled
  label sprite map swap set material.needsUpdate, bumping
  material.version and forcing full program re-resolution next render.
  Swapping between two non-null cached textures never changes the
  compiled program (USE_MAP define unchanged) — new _setLabelMap()
  helper only flags needsUpdate on a null<->texture transition, used at
  all 7 swap sites.
- getBoundingClientRect (~1.2%): the 3D highway's per-frame canvas-size
  self-check forced a layout read every frame. The CSS-box drift read
  now runs every 10th frame (or when the wrap isn't pinned); the
  backing-store comparison stays per-frame with cheap property reads
  and forces an immediate box read + applySize when it fires.
- set textContent: the core 60 Hz HUD clock rewrote hud-time (and
  getElementById'd it) every tick for a display that changes 1/s — now
  write-on-change with a cached element ref.

(The remaining textContent writer in the trace is notedetect's
badges.js — external repo, to be filed there.)

tests/js: resize-reframe shape test updated for the hoisted _bsChanged
gate, incl. an assertion that the throttle can never delay the
backing-store path.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

plugins/highway_3d/plugin.json

@@ -1,7 +1,7 @@
 {
     "id": "highway_3d",
     "name": "3D Highway",
-    "version": "3.31.0",
+    "version": "3.31.1",
     "type": "visualization",
     "bundled": true,
     "script": "screen.js",