perf(feeds): frame-budget governor for camera feeds

[sepahead]

What

Builds on the round-robin camera-feed scheduler (one camera render + pixel readback per tick) with an adaptive frame-budget governor.

The heavy part of a feed tick is a full scene render to the camera's render target plus a synchronous readRenderTargetPixels readback — the op that stalls the main render loop, worst with multi-million-splat scenes. The governor:

• times that heavy work into an EMA (feedCostEmaRef);
• computes stride = clamp(round(cost / FEED_FRAME_BUDGET_MS), 1, MAX_FEED_STRIDE);
• runs the heavy path only every stride ticks, so feeds back off and yield the main thread/GPU under load.

Properties

• No regression at low load — when cost ≤ budget, stride === 1 (every tick), identical to current behavior.
• Smooth motion preserved — the cheap per-tick patrol updates still run every tick; only the heavy render+readback is throttled.
• Bounded worst case — at MAX_FEED_STRIDE = 6 and the 83 ms interval, a feed still refreshes ~every 0.5 s.
• Detection path stays adaptive — when feeds are hidden, the render-to-target cost is still folded into the EMA.

Why

Completes the lag-fix arc (splat performance mode → round-robin feeds → frame-budget governor): the feed subsystem now spends only a bounded slice per tick and gives time back to the render loop precisely when the scene is heavy.

Verification

• bun run typecheck — clean
• eslint — clean (only the pre-existing max-lines warning on this file)
• bun run test:run — 230 passed, 8 skipped

Guarded going forward by scripts/perf-smoke.mjs (splatWithFeeds FPS floor).