feat: dropped-frame / achieved-fps telemetry (fixes #23)

[joeblau]

What & why

Closes #23 (M8 · Dropped-frame / achieved-fps telemetry). There were ≥4 silent frame-shed sites with no counters — PTS-deadline pacing, bufferingNewest(1) backpressure, compositor pool exhaustion, and VideoFrameAdmission — and no measured achieved fps (the HUD showed the encoder's target rate). This adds real observability that feeds the M3 HUD now and is reachable by the M1/M9 adaptive logic.

Changes

• FrameTelemetry (StreamCore) — thread-safe cumulative counters (captured / encoded + the four drop reasons) via OSAllocatedUnfairLock, mirroring VideoFrameAdmission. Plus a pure, unit-tested rate(from:to:elapsed:) for achieved fps + congestion-drops/sec.
• LiveStats gains achievedFrameRate and droppedFrames. Congestion = backpressure + admission (frames the pipeline wanted to encode but couldn't); intentional pacing and overlay-only compositor drops are tracked but excluded from the HUD total.
• Capture output records captured / pacing / backpressure (via the AsyncStream .dropped yield result) / compositor (pool-exhaustion fallback) on the serial sample queue.
• Both publishers record admission + encoded and hold the shared telemetry, so the adaptive controller (M9) can consume the congestion signal.
• Controller folds measured fps + cumulative drops into liveStats each ~1s poll. HUD now shows achieved fps (falling back to the target for the first second) and a subtle "N frames dropped" notice that appears only under real congestion.

Testing

• StreamCoreTests green (144 tests): new FrameTelemetryCounterTests, FrameTelemetryRateTests, and LiveStatsTelemetryTests cover the counters, congestion-vs-total accounting, and the rate math (elapsed-window guard, delta flooring on reset).
• The app-target wiring (capture / publisher / HUD) isn't CI-buildable (needs the iOS 27 SDK); verified by inspection — run on a physical iOS 27 device for the live HUD.

Note: split cleanly off a working tree that also carried in-progress #22 (HEVC / low-latency) work — the diffs here are telemetry-only.

🤖 Generated with Claude Code