Playback evidence: reset stale frames and preserve consent gates

[jlipworth]

Scope

Fixes #311. Capture-enabled debug probes clear all previous frame labels at entry, before admission/readiness failures; reset failure blocks. Pending encoding consent becomes an explicit blocked/consentRequired result, preserving a pending pre-cleanup snapshot and separate stopped snapshot. Offline evidence validation accepts the new consent/reset gate reasons.

Rebased onto main 2930d046 after PR310 landed, replaying only this PR's two commits. PR314 remains separate and unchanged. Production consent, Original/Maximum behavior, Generic profile and decoder policy are unchanged.

Rebased-head validation

• Full hermetic PMSKit suite (1,705 tests), hygiene, strict MkDocs, links and Mermaid checks passed.
• Fresh iPhone build; all five focused hosted evidence tests (including reset and consent) plus semantic fixture-detail smoke passed.
• Normal Plex Link authentication; live successful playback produced eight current frames with initial/post-hold visual review. The immediately following Original encoding-required source reported blocked/consentRequired, retained pending and cleanup snapshots, and left the entire capture directory empty. No encoding approval was supplied.
• Offline parser recognized the real blocked report (expected blocked exit 2), rather than rejecting its reason.
• Fresh visionOS, tvOS and isolated Mac native builds and fixture smokes passed; owned resources cleaned up.

Limits

This is bounded reset/consent evidence, not a rerun of the historical 27-report dependent-stack corpus audit. PR314 track-transition work and physical-device display/audio/HDR acceptance are separate. Private source bindings, frames and diagnostics remain gitignored. No infrastructure changes. Awaiting parent exact-head review/merge; do not merge automatically.