Harden MobileCore v2 model switching

[Harzva]

Summary

• add projected post-switch memory preflight with bounded active-runtime reclamation
• revalidate runtime identity immediately before model load
• expose backend and extended safe metrics in the MobileCore control surface
• record redacted switch-preflight metadata in ActionEvidence
• refresh v0.1.78 dual-app QA evidence

Verification

• 582 Flutter tests passed
• targeted analyzer completed without fatal diagnostics (repository retains historical lint debt)
• public release credential policy and its 4 tests passed
• pureDebug and pureRelease APK builds passed
• Android 16 ARM64 emulator: final APK 30/30 offline cross-app tasks passed (15 buffered, 15 SSE)
• post-run MobileCore protocol v2 health ready; no fatal/ANR/OOM/SIGABRT

Remaining boundary

Physical Android/iOS, verified Omni audio, thermal, and controlled-account acceptance remain open and are not claimed by this PR.