Add FTS5-style command column and runtime oversample for rescore

Replace the old INSERT INTO t(rowid) VALUES('command') hack with a
proper hidden command column named after the table (FTS5 pattern):

  INSERT INTO t(t) VALUES ('oversample=16')

The command column is the first hidden column (before distance and k)
to reserve ability for future table-valued function argument use.

Schema: CREATE TABLE x(rowid, <cols>, "<table>" hidden, distance hidden, k hidden)

For backwards compat, pre-v0.1.10 tables (detected via _info shadow
table version) skip the command column to avoid name conflicts with
user columns that may share the table's name. Verified with legacy
fixture DB generated by sqlite-vec v0.1.6.

Changes:
- Add hidden command column to sqlite3_declare_vtab for new tables
- Version-gate via _info shadow table for existing tables
- Validate at CREATE time that no column name matches table name
- Add rescore_handle_command() with oversample=N support
- rescore_knn() prefers runtime oversample_search over CREATE default
- Remove old rowid-based command dispatch
- Migrate all DiskANN/IVF/fuzz tests and benchmarks to new syntax
- Add legacy DB fixture (v0.1.6) and 9 backwards-compat tests

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

tests/test-general.py

@@ -27,3 +27,15 @@ def test_info(db, snapshot):
     assert exec(db, "select key, typeof(value) from v_info order by 1") == snapshot()
 
 
+def test_command_column_name_conflict(db):
+    """Table name matching a column name should error (command column conflict)."""
+    # This would conflict: hidden command column 'embeddings' vs vector column 'embeddings'
+    with pytest.raises(sqlite3.OperationalError, match="conflicts with table name"):
+        db.execute(
+            "create virtual table embeddings using vec0(embeddings float[4])"
+        )
+
+    # Different names should work fine
+    db.execute("create virtual table t using vec0(embeddings float[4])")
+
+