feat: fail closed on MobileCore background restriction

[Harzva]

Summary

• parse MobileCore's Android background_restricted health state
• preserve model metadata for diagnosis while removing a restricted runtime from eligible local routes
• reject local inference with a typed error before sending prompts or attachments
• show a Battery-settings recovery instruction in the TuiMa control sheet
• update dual-app evidence with the 40-poll Android 16 rc6 background closure

Verification

• focused MobileCore provider/adaptive-policy tests: 42 passed
• full Flutter suite: 578 passed
• targeted analyzer completed without fatal diagnostics (repository-wide baseline still contains pre-existing lint warnings/info)
• pureDebug APK built as 0.1.77+67; local SHA-256 924f223386595332e4bb9b933113619b9423ccf8ce68559bf969084f7ab67b3b
• Semgrep p/secrets: 0 findings

Boundary

MobileCore remains local inference only. MobileCode continues to own Phone Use, transaction risk, one-shot approvals, credentials, cloud consent, and ActionEvidence. Production code does not change Android battery policy automatically.