feat(cache): expose session byte budgets and lifecycle status

[orut34iop]

Hosts can choose a forward window but cannot request an exact temporary VOD cache budget or tell whether cleanup succeeded. Add optional LoadOptions.sessionCacheByteBudget and a separate EngineDiagnostics.sessionCacheStatus snapshot with requested/effective budgets, resident bytes, protected-window overflow, producer parking, eviction and typed cleanup/failure results.

Nil preserves existing automatic sizing. Explicit requests reuse the quarter-of-available-space safeguard and RetentionClaims allocation after other sessions' unwritten claims. Unknown capacity keeps the conservative ceiling. Zero keeps the necessary working window rather than promising zero I/O; the software disk path keeps a bounded working spool. A protected fMP4 playback window may exceed the request, which the snapshot reports explicitly. The tvOS memory default and live DVR policy remain unchanged; live reports this VOD byte policy as unsupported.

State is sampled off-main from the segment ledger at the existing telemetry cadence. Cleanup may publish after stop, but session tokens reject callbacks from a superseded load/rebuild and an old startup callback cannot revive a stopped cache. Remote HLS is reported as AVPlayer/server managed, not successful zero-byte caching. Detailed snapshots cover fMP4 segment caching; software packets retain their existing telemetry.

Keep the existing #720 ENOSPC classification and #714 live DVR limits. A bounded stale-directory sweep retains the liveness marker, reports enumeration/deletion failures, and never reuses another session's bytes. This is not the cross-launch source cache discussed in #706. No UI settings or host-specific cache tiers are included.

This is independent of #827 (explicit packet-store selection), but both touch SoftwarePlaybackHost construction and the LoadOptions inventory. If #827 lands first, the requested-byte resolution belongs inside its disk factory; the memory branch and automatic platform choice must stay unchanged. No combined-branch or device performance acceptance is claimed.

Tests cover default equivalence, explicit/zero/unknown capacity, shared claims, actual segment writes and replacements, parking, hard-window overflow, bounded sweep, failed directory/write/adoption/cleanup, real engine load/stop cleanup and rejection of old-session updates.

Validation: 66 focused tests under the strict cooperative pool passed. Full serial host suite passed: 795 XCTest tests (1 existing skip) and 4,636 Swift Testing tests across 637 suites. Published documentation links and git diff --check passed. Existing Issue451 creation-time/liveness assertions remain unchanged, and a callback-type regression preserves the public zero-argument HLSVideoEngine.stop() API. Platform and optional-trait validation is left to CI; no new device acceptance is claimed.