fix(android): replace destructive Room migration with real migration scaffold + safety export

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Problem

AppDatabase (v3) is built with fallbackToDestructiveMigration(). Any
future schema change — adding a column, renaming a field — silently deletes
every profile on the user's device. Each profile carries the tunnel_key
(AES-256-GCM), script_keys, socks_pass, and remoteUrl. Losing them
requires re-import, and losing the tunnel_key source locks the user out
of their VPN permanently.

What changed

• AppDatabase.kt — flip exportSchema = true, replace
  fallbackToDestructiveMigration() with addMigrations(*ProfileMigrations.ALL).
  Missing migration now crashes instead of silently wiping data.
• ProfileMigrations.kt (new) — SafetyExportMigration scaffold: reads
  all profile rows and writes a JSON snapshot to cacheDir before any
  schema change. If the export fails, Room throws IllegalStateException
  instead of proceeding with data loss. ALL = arrayOf() is empty until
  the first real v3→v4 migration is authored.
• ProfileDao.kt — added getAllOnce() for non-flowing snapshot access.
• build.gradle.kts — ksp { arg("room.schemaLocation", ...) } for
  schema export, androidTestImplementation deps for future migration
  tests.
• ProfilesViewModelParseTest.kt — override getAllOnce() in
  NoopProfileDao test fake (required after interface expansion).

Verification

• :app:assembleDebug — CI green on push ✓
• :app:testDebugUnitTest — CI green on push ✓
• fallbackToDestructiveMigration grep on src/main/ → 0 matches ✓
• Step 6 (androidTest migration test) skipped — no emulator available;
  deferred until emulator-backed CI is set up.