Fix Emby AV1 cold-start deadline teardown cycle

[jlipworth]

Summary

• Mitigate Emby AV1: startup media-response timeouts and repeated encode attempts #305's reproduced cold AV1 encode / CoreMedia deadline / server teardown cycle by prewarming the same Emby session before AVPlayer attaches.
• Scope the 20-second soft warm-up to AV1 transcodes at absent/zero resume. Direct/copy, other codecs, positive-offset handling, Jellyfin, and existing retry limits are unchanged.
• Add fail-closed expected item/source identity assertions to the Emby probe and pure regression tests.

Evidence and validation

• Exact item/source baseline failed before video; warm control ready in ~6.6s.
• Shipping-path candidate and clean-build repeat passed bounded seek-to-zero/hold checks with four initial and four post-hold visible frames; no remaining server encoder after stop.
• Forced early preparation timeout stopped during warm-up; no late item attachment or remaining encoder.
• 1,700 hermetic PMSKit tests; hygiene, strict MkDocs, links/anchors, Mermaid checks passed.
• Fresh iOS, visionOS, tvOS and isolated macOS builds; iPhone browse, visionOS fixture, tvOS/macOS signed-out smoke inspected. Other-platform smokes are not AV1 live-playback acceptance.

Review notes

• Reconciled with main after Fix Jellyfin HEVC HLS packaging and add private corpus testing workflow #306 landed using a merge commit (no history rewrite). Current canonical corpus guidance and Jellyfin implementation/tests are retained unchanged; the obsolete duplicate corpus plan is removed. Net diff is eight AV1/identity-guard files.
• Fresh post-integration exact-bound AV1 repeat reached warm readiness in 7.4s, passed with 4+4 inspected visible frames, and repeated the early-cancellation cleanup check.
• Emby HEVC 10-bit SDR: block/smear corruption after Original to 8 Mbps #304 remains a separate HEVC visual investigation; Jellyfin HEVC audio-only HLS playback at a quality ceiling: validate container and decoded frames #303 stays Jellyfin-only. HDR paused.
• Physical-device, deep-seek, server-version and release gates remain open. No automatic issue closure or merge.
• Raw identifiers, media names, paths, addresses, credentials, and frames remain private and are not attached.