Ice scene: the glossy orbs become rising bubbles, ~38% more transparent

- Orbs rise from below the bottom edge to above the top edge, each with
  its own 36-68s pass (slower on phones), a negative delay (scene already
  populated on load), own horizontal position, a gentle sway and a slight
  scale drift; opacity fades in/out at the edges so the loop never pops.
- Only transform + opacity animate, each orb on its own composited layer
  (will-change: transform); no filter/backdrop-filter, nothing animates
  size/offsets/shadow. The glacier blobs are untouched.
- Opacity lowered ~38% in both themes: 0.55->0.34 light, 0.8->0.5 dark,
  phone overrides 0.32->0.2 / 0.42->0.26.
- prefers-reduced-motion: rise removed, orbs return to the old static
  placements (a paused-from-start CSS animation is not applied in Chrome
  and would park them below the fold); tab hidden: a visibility listener
  parks the whole scene (.ice-paused), blob drift included.
- Measured with a CDP perf trace on a 4x throttled phone profile: p95
  33.4ms, 0 frames over 34ms, no long tasks - identical to the pre-change
  baseline; ~0.6% extra main-thread CPU. Desktop stays compositor-bound
  from the pre-existing blob drift, as before. tests/bubble-perf.mjs
  keeps it repeatable.
- New bubbles suite (34 checks) in the repo harness; the axe contrast
  matrix is now 16 runs (360/390/1280 x dark/light x EN/CS) and passes.
- SW cache -> freezmo-v31; README decision 46.

Full suite: 11/11 suites, 424 checks.

tests/README.md

@@ -27,6 +27,7 @@ prints a per-suite summary at the end.
 | Suite | Covers |
 | --- | --- |
 | `regression.mjs` | Library, deck card + menu, study feed to results, homework entry, desktop shell + settings/health panel. |
+| `bubbles-test.mjs` | Ice-scene bubbles: stacking/layout contract, keyframes only transform+opacity, rise/sway/scale/fade phases, per-breakpoint counts (5/3/2), phone pacing, reduced motion, tab-hidden parking. |
 | `mobile-test.mjs` | Five-tab bar at 360/390 × EN/CS × dark/light × reduced motion (labels, targets, no hamburger/drawer), the library ⋯ sheet (Import JSON / New folder), the settings sheet (touch targets, key flow, drag-to-close, backdrop), axe contrast audits. |
 | `wizard-test.mjs` | Deck 3-step flow and homework flow, flat action bar, in-page mode switch, the loading lock, abort-on-leave. |
 | `native-test.mjs` | Manifest/PWA head, offline banner + disabled generate, share target GET + POST, Back-gesture layers, undo toast, tab re-tap, haptics, draft restore. |
@@ -44,3 +45,10 @@ stub Ollama (or VAPID keys) on a spare port (3208/3209). Each suite checks the
 port is free first and kills the whole process group on exit, so ports do not
 leak between suites. Scratch data for the VAPID test lives in `tests/.tmp/`
 (gitignored).
+
+## Diagnostics (not part of the run)
+
+`node tests/bubble-perf.mjs [label]` measures the ice-scene frame cost with a
+CDP performance trace (rAF frame timing, long tasks, main-thread busy time,
+style recalcs) on a 4× throttled phone profile and a 2× desktop profile. It
+always exits 0; the numbers are for tuning the bubble count/speed.