feat(02-05): wire compost beat toast (Plan 02-04 deferral)

- src/ui/settings/compost-toast.tsx: thin transient toast (D-07,
  GARD-04). Reads a rotating line from uiStrings[1].post_harvest_beat
  on each compost dispatch; fades out after 3.5s. Co-located with
  PersistenceToast as the plan specified ('folded into the persistence-
  toast UI surface').
- src/store/session-slice.ts: compostBeatTick monotonic counter +
  bumpCompostBeat action. Counter (vs boolean) ensures consecutive
  composts re-fire the toast without dedup.
- src/game/scenes/Garden.ts: handleTilePointerDown's compost branch
  calls bumpCompostBeat after enqueueCommand.
- src/App.tsx: mounts <CompostToast />.
- 4 new compost-toast tests; 312/312 vitest green; e2e still 1.6s
  green; npm run ci exits 0.

Implementation choice (per plan 'surfaced in SUMMARY'): minimum-viable
toast surface chosen over the Ink runtime path. The Ink-authored
compost-acknowledgements.ink content remains compiled + runtime-
loadable via loadInkStory + InkRenderer, so a future plan can swap
this component for the richer voice without touching sim or store.

src/ui/settings/index.ts

@@ -4,6 +4,10 @@
  * Plan 02-05 ships:
  *   - Settings: D-28 save-management modal (Export / Import / Restore)
  *   - PersistenceToast: D-30 one-time soft toast in voice
+ *   - CompostToast: D-07 + GARD-04 thin transient toast (Plan 02-04
+ *     deferral landed here — co-located with PersistenceToast since
+ *     both share the same toast UX shape).
  */
 export { Settings } from './Settings';
 export { PersistenceToast } from './persistence-toast';
+export { CompostToast } from './compost-toast';