Moves message history, contacts and discovered contacts out of the settings
store. Settings stay in SharedPreferences, which is what it is for.
Ordering is the whole safety argument: WRITE, VERIFY BY READING BACK, and only
then remove the source. #333 was a storage path that chose a key silently and
made 566 real messages read as empty; deleting before verifying would make
that class of mistake permanent instead of cosmetic. On any failure the source
is left intact and the error is logged - never a silent drop (SAFELANE 6).
Idempotent by construction: a key already present in drift is not overwritten,
so re-running is a no-op. If an older build re-writes a migrated key into
prefs, the migrated copy wins and the stale prefs copy is discarded rather
than promoted.
Rehearsed against a COPY of a real 7 MB store, as the plan required before
touching live data:
REHEARSAL: 69 migrated, 0 failed, 5.21 MB, 69 bulk keys expected
Every key checked for exact length, confirmed removed from prefs, and every
settings key confirmed untouched. The live store was never opened.
Two things the tests caught that review would not have:
1. getString THROWS on a non-string value rather than returning null, so a
non-string under a bulk prefix was counted as a migration FAILURE. It now
type-checks with prefs.get() and skips. Alarming falsely is its own bug.
2. The `contacts` prefix was checked against the real store rather than
assumed: it matches only the 6 bulk contact blobs, and correctly does NOT
match contact_unread_count*.
Not yet wired into app startup - that is the switchover, and it is deliberately
a separate commit so this can be reviewed on its own.
flutter analyze clean, dart format clean, 504 tests pass.
Strycher
a2c7302aad