fix: peer review — 5 correctness bugs across who, db, lock, embedding, main

Comprehensive peer code review identified and fixed the following:

1. who.rs: @-prefixed path routing used `target` (with @) instead of
   `clean` (stripped) when checking for '/' and passing to Expert mode,
   causing `lore who @src/auth/` to silently return zero results because
   the SQL LIKE matched against `@src/auth/%` which never exists.

2. db.rs: After ROLLBACK TO savepoint on migration failure, the savepoint
   was never RELEASEd, leaving it active on the connection. Fixed in both
   run_migrations() and run_migrations_from_dir().

3. lock.rs: Multiple acquire() calls (e.g. re-acquiring a stale lock)
   replaced the heartbeat_handle without stopping the old thread, causing
   two concurrent heartbeat writers competing on the same lock row. Now
   signals the old thread to stop and joins it before spawning a new one.

4. chunk_ids.rs: encode_rowid() had no guard for chunk_index >= 1000
   (CHUNK_ROWID_MULTIPLIER), which would cause rowid collisions between
   adjacent documents. Added range assertion [0, 1000).

5. main.rs: Fallback JSON error formatting in handle_auth_test
   interpolated LoreError Display output without escaping quotes or
   backslashes, potentially producing malformed JSON for robot-mode
   consumers. Now escapes both characters before interpolation.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/embedding/chunk_ids.rs

@@ -1,6 +1,10 @@
 pub const CHUNK_ROWID_MULTIPLIER: i64 = 1000;
 
 pub fn encode_rowid(document_id: i64, chunk_index: i64) -> i64 {
+    assert!(
+        (0..CHUNK_ROWID_MULTIPLIER).contains(&chunk_index),
+        "chunk_index {chunk_index} out of range [0, {CHUNK_ROWID_MULTIPLIER})"
+    );
     document_id
         .checked_mul(CHUNK_ROWID_MULTIPLIER)
         .and_then(|v| v.checked_add(chunk_index))