Repairs six defects found while reviewing the full release/2.8.0 branch against main. Three would have shipped a feature broken or unsafe in production in a way CI cannot catch; the rest fix a silently-dead search backfill, restore suppressed clean-air telemetry, and add the regression coverage that was missing for these paths.

🐛 Bug Fixes

• Car App could not connect to Android Auto in any release build (feat(car): Android Car App Library integration #5633). MeshtasticCarAppService.createHostValidator() built the HostValidator from a car_hosts_allowlist of bare package names, but androidx.car.app's addAllowedHosts() requires digest,packageName entries and threw IllegalArgumentException on the first one during the host handshake (masked locally by the debug ALLOW_ALL_HOSTS_VALIDATOR). Replaced with Google's official entries, copied verbatim from androidx.car.app:1.9.0-alpha01's bundled hosts_allowlist_sample (3 Android Auto gearhead keys + 3 AAOS templates.host keys). ⚠️ Security-sensitive — worth a sanity check before merge.
• AppFunctions emitted malformed node IDs for ~half the node-num space (feat(ai): Add App Functions for system AI integration #5585). AiFunctionProviderImpl used "!${node.num.toString(16)}"; for high-bit (negative Int) node nums this produced !-1 instead of the canonical !ffffffff, and the reverse toInt(16) lookup threw for canonical IDs greater than Int.MAX. Switched to the existing NodeAddress.numToDefaultId / idToNum helpers.
• Discovery scan abort stranded the radio on the scan preset (feat(discovery): mesh network discovery #5275). pauseAndAbort() cancelled its own scanScope before restoring the home LoRa preset, so the restore/finalize threw CancellationException and never ran — leaving the radio on the scan modem preset with the UI stuck on Failed. Reordered to finalize and reach the terminal state, restore on applicationScope, then cancelScanInternal() last (mirroring stopScan()).
• FTS5 historical-message backfill was dead code (feat: FTS5 full-text message search #5373). It keyed on json_extract(data, '$.text'), but DataPacket.text is a computed property never serialized into the stored JSON (the body is persisted as bytes), so the backfill silently no-opped and every message received before the v39 upgrade stayed permanently unsearchable. Replaced with a Kotlin-side decode that writes message_text via the previously-unused updateMessageText.
• Air-quality summary cards hid legitimate 0 readings (feat: add air quality telemetry display (PM1.0, PM2.5, PM10, CO2) #5701). Dropped the != 0 guards on the node-detail cards (kept the ?.let null/presence check), consistent with the fix(node): chart local-node air-quality telemetry (orphaning + zero-suppression) #5793 chart/CSV zero-suppression fix.

🧹 Chores

• Removed the now-unused HEX_RADIX constant from AiFunctionProviderImpl.

🧪 Testing Performed

• AiFunctionProviderImplTest — added getNodeDetails_round_trips_high_bit_node_num (canonical ID round-trip for high-bit nums).
• PacketFtsSearchTest (new, core:database androidHostTest) — search match/non-match, conversation scoping, and the backfill regression (the new test fails against the old json_extract implementation).
• DiscoveryScanEngineTest (feature:discovery commonTest) — reconnect-timeout abort reaches Complete(Failed), finalizes the session as failed, and restores the home preset, plus stopScan and normal-completion restore. FakeRadioController now records setLocalConfig calls so the restore can be asserted.
• Full local baseline: spotlessCheck detekt assembleDebug test allTests kmpSmokeCompile → BUILD SUCCESSFUL (includes the KoinVerificationTest typed-bootstrap graph).

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