fix(state): do not stamp empty FTS after interrupted optimize-storage demote

Demote wrote the empty v23 schema via executescript inside BEGIN IMMEDIATE,
which commits early and can leave trash + empty indexes without rebuild
markers. Re-run then tore down trash and stamped fts_storage_version with
docsize=0, permanently losing historical session search.

Stage markers with the demote, create schema only after they are durable,
heal empty-index bookkeeping on resume, and refuse settle until the base
index is populated. Settle refusal returns ok=False instead of raising,
and resume fails fast if the base v23 table cannot be re-created.

Orphan-marker repair only resets a missing fts_rebuild_progress to 0 once
the index is known empty: the chunk worker replays its whole selected id
range without an anti-join, so a partially indexed DB that lost only its
progress key is first reset to a known-empty surface, then rebuilt.

Ported onto the SessionDB mixin split (hermes_state_search.py /
hermes_state_schema.py).

hermes_state_schema.py

@@ -730,7 +730,8 @@ class SessionSchemaMixin:
             # FTS_STORAGE_VERSION; a legacy DB is left at whatever it had
             # (absent/0) until `optimize-storage` runs. An INTERRUPTED
             # optimize (legacy vtables already demoted, but rebuild markers
-            # or demoted trash tables still present) is NOT stamped either —
+            # or demoted trash tables still present, or an empty external
+            # index against non-empty messages) is NOT stamped either —
             # the marker is the source of truth for "fully optimized", and
             # `fts_optimize_available()` keeps offering the resume until the
             # transition actually completes.
@@ -742,6 +743,7 @@ class SessionSchemaMixin:
                     "WHERE key = 'fts_rebuild_high_water' LIMIT 1"
                 ).fetchone() is None
                 and not self._has_fts_trash(cursor)
+                and not self._fts_external_index_empty_with_messages(cursor)
             ):
                 self.set_meta(
                     "fts_storage_version", str(FTS_STORAGE_VERSION), cursor=cursor