feat(fonts): bounded late-load reflow scheduler

[caio-pizzol]

Stacked on caio/font-load-planner (PR #3612) — review that first; this diff is the scheduler only.

The face-aware planner (#3612) narrows loading to the faces a document renders. This adds the other half of making a large font pack safe: it stops a font-heavy document on a slow network from triggering a full re-measure on every late font wave.

When a required face loads after the gate's first-paint timeout, the document must re-measure so it stops rendering against a fallback. On a slow link a font-heavy doc's faces arrive in many waves (a scaling probe measured ~38 over ~103s for 40 fonts) — reflowing per wave is a re-measure storm. FontReadinessGate now batches late-loaded faces through a small scheduler.

Policy: leading flush + throttled trailing (cooldown). The first late face flushes after a short quiet window (coalescing the initial parallel batch). After any flush a cooldownMs window opens during which further arrivals are deferred; at its end one trailing flush drains them, then the cooldown reopens if more arrive. This bounds the flush rate to ~once per cooldown regardless of arrival spacing — unlike a plain debounce (one flush per wave when waves are spaced wider than the window) or a per-batch max-wait (the quiet flush resets it before it bites). Honest floor: arrivals spaced wider than cooldownMs reflow per arrival — you can't coalesce a wave that lands after the doc already corrected without delaying every correction by at least the gap, so cooldownMs is the max correction lag.

What changed

• FontLateLoadReflowScheduler.ts: the quiet + cooldown policy; injectable timers; schedule() / flushNow() / cancel().
• FontReadinessGate: #onLoadingDone records the changed required-face keys and calls scheduler.schedule(...); one flush does bump-epoch + invalidate + reflow. notifyFontConfigChanged reflows immediately and cancels any pending batch (so a stale batch can't fire a second reflow right after). dispose() cancels pending work so a torn-down editor never reflows.

Out of scope (unchanged)

• First paint is still bounded by the 3s per-font gate timeout — the scheduler only governs after-the-fact corrections.
• No public font API change; no incremental block-level re-measure; no font pack.

Verified

• Repo typecheck clean. Unit tests: scheduler policy — burst coalescing, 40 arrivals 500ms apart → far fewer than 40 flushes (the spaced-wave bound), trailing-flush-after-cooldown, dedup, cancel, flushNow, fresh-batch-after-idle; gate integration — batched late loads → one reflow, irrelevant face → none, config-change cancels a pending batch (no double reflow), dispose cancels, notifyFontConfigChanged immediate. Run in CI.
• Browser: with Carlito delayed past the gate timeout, first paint happens with fallback (not blocked) and the late load produces a single batched reflow.

[cubic-dev-ai]

No issues found across 4 files

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[codecov-commenter]

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