fix(state): align messages_fts external content with its indexed projection

`messages_fts` declared `content='messages'` while its triggers indexed only a
bounded prefix of every long tool row, with the boundary held in a
`fts_tool_full_content_high_water` state_meta marker that the migration, the
rebuild seeding and the in-place rebuild path all re-stamped. FTS5's strict
integrity check re-reads the external content source and compares it with the
stored token stream, so the two could never agree: one long tool row is enough to
make

    INSERT INTO messages_fts(messages_fts, rank) VALUES('integrity-check', 1)

fail with `fts5: checksum mismatch for table "messages_fts"`. The delete/update
triggers re-evaluated the *new* marker, so they sent full content for a row whose
index held a prefix and left tokens behind that survived deleting the row.

Fix the class by removing the moving part. `messages_fts` now reads a view,
`messages_fts_src`, that computes exactly what the writers index - tool rows
truncated to FTS_TOOL_CONTENT_PREFIX_CHARS, everything else verbatim - through a
fixed per-row expression with no state_meta lookups, shared by the triggers, the
boundary sweep and the chunked insert.

- `messages_fts_src` view added; `messages_fts` external content points at it
- triggers, boundary sweep and chunked backfill all read that one projection
- `_stamp_fts_tool_high_water`, the marker seeding in `_seed_fts_rebuild_markers`
  and the in-place rebuild stamp are gone, with
  FTS_TOOL_FULL_CONTENT_HIGH_WATER_KEY
- FTS_STORAGE_VERSION 2 -> 3: an existing index is re-pointed and rebuilt ONCE by
  `_migrate_misaligned_fts_source` under the shared cross-process rebuild
  admission, because a v2 index holds token streams its old source cannot read
  back; an empty index swaps shape in place, and legacy inline DBs are untouched
- search behaviour is unchanged: tool rows were already prefix-indexed, and
  explicit tool search already used the stored-content LIKE path

hermes_state_common.py

@@ -253,23 +253,27 @@ AUTO_VACUUM_MIN_FREELIST_RATIO = 0.25
 # layout 0 (marker absent) with a working inline index until the user opts in.
 #   1 = v23 external-content layout with a tool-row-excluded trigram
 #   2 = trigram also excludes structured tool_calls JSON
-FTS_STORAGE_VERSION = 2
+#   3 = messages_fts source aligned to a stable projection view
+#       (``messages_fts_src``): always-truncate tool rows to the prefix, no
+#       moving high-water boundary. The external-content source now reads
+#       back EXACTLY what the triggers indexed, so the rank=1
+#       'integrity-check' probe cannot drift from the stored index (the
+#       recurring fts5 "checksum mismatch" / leaked-token failures).
+FTS_STORAGE_VERSION = 3
 
-# Tool results are often multi-megabyte machine payloads. Index a useful
-# prefix for new tool rows instead of tokenizing the entire body while the
-# canonical message write holds SQLite's single writer lock. The high-water
-# marker lets upgraded databases retain the exact token stream already stored
-# for historical rows, so external-content delete/update commands stay valid
-# without an eager full-index rebuild.
+# Tool results are often multi-megabyte machine payloads. The base FTS index
+# stores only a bounded prefix of every tool row; tool rows are skipped by
+# default in search, and explicit tool-only search uses a LIKE fallback over
+# the full stored content, so no search capability is lost. The projection
+# below is STABLE — it depends only on the row being written, never on
+# mutable ``state_meta`` markers — which is what keeps the external-content
+# integrity checker and the trigger 'delete'/'update' commands in agreement
+# with the stored index forever.
 FTS_TOOL_CONTENT_PREFIX_CHARS = 8_192
-FTS_TOOL_FULL_CONTENT_HIGH_WATER_KEY = "fts_tool_full_content_high_water"
 
 
 def _fts_indexed_content_sql(alias: str) -> str:
     return f"""CASE WHEN {alias}.role = 'tool'
-              AND {alias}.id > COALESCE((SELECT CAST(value AS INTEGER)
-                                         FROM state_meta
-                                         WHERE key = '{FTS_TOOL_FULL_CONTENT_HIGH_WATER_KEY}'), -1)
          THEN substr(COALESCE({alias}.content, ''), 1, {FTS_TOOL_CONTENT_PREFIX_CHARS})
          ELSE {alias}.content END"""
 
@@ -691,12 +695,32 @@ CREATE INDEX IF NOT EXISTS idx_sessions_effective_activity
 # predicate into a tautology (id > -1 OR id <= -1), i.e. normal operation.
 # The two state_meta PK probes per write are negligible next to the FTS
 # insert itself.
+#
+# messages_fts_src: the base word index no longer reads raw `messages` as its
+# external content. Tool rows are indexed as a bounded prefix, so the index
+# must read that SAME projection back or FTS5's 'integrity-check' / 'delete'
+# commands disagree with the stored tokens and corrupt the index (the
+# recurring fts5 checksum-mismatch drift: the projection used to depend on a
+# moving state_meta high-water key). The view/trigger/backfill all share the
+# one expression in `_fts_indexed_content_sql` — a fixed per-row function
+# with no marker lookups — so the boundary can never move again.
 FTS_SQL = f"""
+-- Stable projection the base word index reads and writes through: the view
+-- computes EXACTLY what the triggers/backfill insert, so 'rebuild' and the
+-- integrity checker always agree with the stored index.
+CREATE VIEW IF NOT EXISTS messages_fts_src AS
+    SELECT id,
+           CASE WHEN role = 'tool'
+                THEN substr(COALESCE(content, ''), 1, {FTS_TOOL_CONTENT_PREFIX_CHARS})
+                ELSE content END AS content,
+           tool_name, tool_calls
+    FROM messages;
+
 CREATE VIRTUAL TABLE IF NOT EXISTS messages_fts USING fts5(
     content,
     tool_name,
     tool_calls,
-    content='messages',
+    content='messages_fts_src',
     content_rowid='id'
 );
 
@@ -902,7 +926,9 @@ CREATE VIRTUAL TABLE IF NOT EXISTS messages_fts USING fts5(
 CREATE TRIGGER IF NOT EXISTS messages_fts_insert AFTER INSERT ON messages BEGIN
     INSERT INTO messages_fts(rowid, content) VALUES (
         new.id,
-        COALESCE({_FTS_NEW_INDEXED_CONTENT_SQL}, '')
+        COALESCE(CASE WHEN new.role = 'tool'
+                      THEN substr(COALESCE(new.content, ''), 1, {FTS_TOOL_CONTENT_PREFIX_CHARS})
+                      ELSE new.content END, '')
         || ' ' || COALESCE(new.tool_name, '') || ' ' || COALESCE(new.tool_calls, '')
     );
 END;
@@ -916,7 +942,9 @@ AFTER UPDATE OF content, tool_name, tool_calls, role ON messages BEGIN
     DELETE FROM messages_fts WHERE rowid = old.id;
     INSERT INTO messages_fts(rowid, content) VALUES (
         new.id,
-        COALESCE({_FTS_NEW_INDEXED_CONTENT_SQL}, '')
+        COALESCE(CASE WHEN new.role = 'tool'
+                      THEN substr(COALESCE(new.content, ''), 1, {FTS_TOOL_CONTENT_PREFIX_CHARS})
+                      ELSE new.content END, '')
         || ' ' || COALESCE(new.tool_name, '') || ' ' || COALESCE(new.tool_calls, '')
     );
 END;
@@ -932,7 +960,9 @@ CREATE VIRTUAL TABLE IF NOT EXISTS messages_fts_trigram USING fts5(
 CREATE TRIGGER IF NOT EXISTS messages_fts_trigram_insert AFTER INSERT ON messages BEGIN
     INSERT INTO messages_fts_trigram(rowid, content) VALUES (
         new.id,
-        COALESCE({_FTS_NEW_INDEXED_CONTENT_SQL}, '')
+        COALESCE(CASE WHEN new.role = 'tool'
+                      THEN substr(COALESCE(new.content, ''), 1, {FTS_TOOL_CONTENT_PREFIX_CHARS})
+                      ELSE new.content END, '')
         || ' ' || COALESCE(new.tool_name, '') || ' ' || COALESCE(new.tool_calls, '')
     );
 END;
@@ -946,7 +976,9 @@ AFTER UPDATE OF content, tool_name, tool_calls, role ON messages BEGIN
     DELETE FROM messages_fts_trigram WHERE rowid = old.id;
     INSERT INTO messages_fts_trigram(rowid, content) VALUES (
         new.id,
-        COALESCE({_FTS_NEW_INDEXED_CONTENT_SQL}, '')
+        COALESCE(CASE WHEN new.role = 'tool'
+                      THEN substr(COALESCE(new.content, ''), 1, {FTS_TOOL_CONTENT_PREFIX_CHARS})
+                      ELSE new.content END, '')
         || ' ' || COALESCE(new.tool_name, '') || ' ' || COALESCE(new.tool_calls, '')
     );
 END;