perf(engine): one-pass VFR extraction with -fps_mode cfr (#1899)

* perf(engine): write PNG frames at compression_level 1

Extracted video frames are render-scoped temp files read once during
capture, so zlib effort above level 1 buys nothing. Measured 3.3x
faster on 60s of 1080p H.264 to PNG (11.4s to 3.5s) and 5.4x on a 20s
vp9-alpha webm (4.3s to 0.79s), for ~14% larger temp files.

* perf(engine): one-pass VFR extraction with -fps_mode cfr

VFR sources (screen recordings, phone videos) were re-encoded to CFR
with libx264 and then extracted in a second ffmpeg pass. Extraction now
runs a single pass with -fps_mode cfr -r <fps>. Same frame counts on
the VFR regression fixtures (120/120 mid-seek, 297-303 full file), one
less x264 generation of quality loss, ~3.4x faster on VFR inputs.
convertVfrToCfr and the _vfr_normalized intermediate are deleted.

The full-VFR test's byte-identical duplicate-frame cap is retired with
cause: the fixture has no source frames for 40% of its timeline, so
held frames are correct; the two-pass path only scored under it because
x264 encoder noise made frozen frames hash differently. The freeze
regression (missing frames) stays pinned by the frame-count windows.

* docs(engine): pin vfrPreflightMs definition change after one-pass VFR

vfrPreflightMs used to time a per-source VFR-to-CFR re-encode; it now
times only the cached classification probe and collapses to ~0. Call
that out on ExtractionPhaseBreakdown so dashboards keyed on the old
threshold semantics migrate to vfrPreflightCount / extractMs.

* fix(engine): bump extraction cache schema to v3 for one-pass VFR frames

One-pass VFR extraction changes frame CONTENTS for VFR sources while
the cache key tuple (path, mtime, size, trim, fps, format) is
unchanged, so warm v2 entries holding two-pass frames would keep being
served across the deploy boundary. Bumping the schema prefix makes v2
entries inert; affected sources re-extract once.

packages/engine/src/services/extractionCache.test.ts

@@ -33,7 +33,7 @@ const keyFor = (videoPath: string, overrides: Partial<CacheKeyInput> = {}): Cach
 
 describe("extractionCache constants", () => {
   it("exposes the v2 schema prefix", () => {
-    expect(SCHEMA_PREFIX).toBe("hfcache-v2-");
+    expect(SCHEMA_PREFIX).toBe("hfcache-v3-");
   });
 
   it("exposes the frame filename prefix shared with the extractor", () => {