fix(core): eliminate background_downloader temp-file leak + deterministic task IDs (#383)

[DenisovAV]

What

Failed / cancelled / process-killed large-file model downloads leaked multi-GB background_downloader temp files (com.bbflight.background_downloader<rand>) in Android filesDir — persistent internal storage the OS never reclaims. Reported in #383 with an exceptionally detailed source-level breakdown by @alan-insam. Fixes all five findings plus the migration needed to make the task-ID change regression-free.

Changes

• Wrong typo in README #1 group mismatch — one SmartDownloader.downloadGroup constant; every cleanup / resume call site routed through it (three used the stale flutter_gemma_downloads literal ≠ the smart_downloads group downloads run under — the leak amplifier).
• iOS: Build Error #2 non-deterministic task IDs — sha256 of the stable Task.split triple (baseDirectory / directory / filename), not hashCode and not the URL. Stable across process restarts, signed-URL rotation, and iOS app-container UUID churn.
• <uses-native-library> missing in Android LLM Inference Guide #3 / Putting model inside output_path folder does not work #6 invisible fragments — getOrphanedFiles() now scans filesDir for com.bbflight.background_downloader* temps (flagged isDownloadFragment); cleanupStorage() deletes them.
• Size concern  #4 watchdog leak — the resume watchdog now cancels the native task and purges its resume/paused/db records before failing; the attach path calls resume() on a recovered paused task instead of stalling until timeout.
• Failed to build iOS app #5 reset vs cancel — cleanup cancels tasks (which deletes paused temp files) before reset(); performCleanup whole-group, _cleanupAllTasksOfType type-scoped.

Anti-regression migration

Legacy resume records written by a pre-1.3.1 version are reconciled and purged at startup before the reclaim gate — a resumeFromBackground()-materialized paused record no longer wedges it — and the blanket sweep only skips when a task is genuinely running (nativeRunningIds = allTasks − pausedTasks), with a 10-min mtime guard against a just-rescheduled download.

Behaviour notes

• The plugin now drives the process's first resumeFromBackground().
• A partial from an older plugin version is reclaimed on next launch (its record is reconciled), not silently stranded.
• Android-only: reclaim + fragment scan are Platform.isAndroid-gated; iOS/macOS/web unaffected (the id-scheme change on iOS is a net improvement — stable across container-UUID rotation).

Verification

• Host-VM: flutter analyze 0 errors/warnings, flutter test 470/470, flutter build web clean.
• Cross-platform compile: iOS, macOS, and Android --release all build clean.
• Device (Firebase Test Lab, Galaxy S23 Ultra / Android 14): 4/4 tests pass — orphan-fragment cleanup and the legacy-record reconciliation verified against real background_downloader storage.

Targets flutter_gemma 1.3.1. Pins background_downloader ^9.5.6.

Fixes #383