docs: fix seven defects in the own-store plan

Reviewed the plan against the code it describes. Two design holes and
five errors.

Instance identity was the real one. The plan deleted the materialised
instances table without saying what replaces the instance row id, which
TasksRepositoryImpl.updateTask passes to updateInstance and which
ListsScreen keys a lazy list by. Two occurrences of one series can
appear in the same list, so taskId alone is not unique and a hash of
(taskId, start) can collide - as a Compose key that is a visible bug.
Task.id is dropped for occurrenceStart, updateInstance takes
(taskId, occurrenceStart, form), and External mode maps back to a real
instance row with one query. This is the single seam change, and the
plan's "TasksDataSource unchanged" claim was wrong.

Local lists had no account name. TaskList.accountName is non-null,
ListsViewModel groups by it and ListsScreen renders it as a section
header, so a null account_id must still report "Local".

The unique index was wrong: overrides share their master's UID, so
unique (list_id, uid) would reject the rows the recurrence design
depends on. It needs recurrence_id in the key.

Phase 0 broke background reminders. It dropped our authority from
ProviderChangeReceiver's manifest filter while the provider was still
the store, and renamed StorageMode.LOCAL to OWN four phases before OWN
meant Room. Both moved to phase 5.

Parity against the provider was overclaimed: the provider materialises
one occurrence, so multi-occurrence expansion has nothing to compare
against and is tested against RFC 5545 directly.

The phases sum to 6.5-7 weeks, not the 6-6.5 stated, and the difference
from STORAGE-DECISION.md's 4.5-6 is now explained rather than left as a
contradiction.

Gaps closed: WAL vs Auto Backup (checkpoint on ON_STOP, sidecars in the
backup rules, tested in phase 6), cascade rules for master_id and
parent_id, Instant type converters, a rollback path that re-runs the
import from tasks.db.imported, the release note for dropping the
authority and its permissions, and ICalendarWriter.uidFor's synthesis
branch becoming External-only.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

docs/STORAGE-DECISION.md

@@ -143,6 +143,12 @@ Two things make instance expansion less frightening than its line count:
 | Tests to parity with the current 93 + 56 | 1 wk |
 | **Net additional** | **4.5–6 wk** |
 
+> ⚠️ Superseded by [`OWN-STORE.md`](OWN-STORE.md)'s **6.5–7 wk**. The figure
+> above costed the new store only, on the assumption `:provider` would be kept
+> beside it. The decision taken was to delete the provider, which adds the
+> untangling (phase 0) and the removal (phase 5) that this estimate never had to
+> include. The costing logic stands; the total does not.
+
 ### What it removes from the sync plan
 
 Roughly sixteen of the phase-1 audit's storage findings are **provider-imposed**