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/generate_legacy_db.py

@@ -0,0 +1,81 @@
+# /// script
+# requires-python = ">=3.10"
+# dependencies = ["sqlite-vec==0.1.6"]
+# ///
+"""Generate a legacy sqlite-vec database for backwards-compat testing.
+
+Usage:
+  uv run --script generate_legacy_db.py
+
+Creates tests/fixtures/legacy-v0.1.6.db with a vec0 table containing
+test data that can be read by the current version of sqlite-vec.
+"""
+import sqlite3
+import sqlite_vec
+import struct
+import os
+
+FIXTURE_DIR = os.path.join(os.path.dirname(__file__), "fixtures")
+DB_PATH = os.path.join(FIXTURE_DIR, "legacy-v0.1.6.db")
+
+DIMS = 4
+N_ROWS = 50
+
+
+def _f32(vals):
+    return struct.pack(f"{len(vals)}f", *vals)
+
+
+def main():
+    os.makedirs(FIXTURE_DIR, exist_ok=True)
+    if os.path.exists(DB_PATH):
+        os.remove(DB_PATH)
+
+    db = sqlite3.connect(DB_PATH)
+    db.enable_load_extension(True)
+    sqlite_vec.load(db)
+
+    # Print version for verification
+    version = db.execute("SELECT vec_version()").fetchone()[0]
+    print(f"sqlite-vec version: {version}")
+
+    # Create a basic vec0 table — flat index, no fancy features
+    db.execute(f"CREATE VIRTUAL TABLE legacy_vectors USING vec0(emb float[{DIMS}])")
+
+    # Insert test data: vectors where element[0] == rowid for easy verification
+    for i in range(1, N_ROWS + 1):
+        vec = [float(i), 0.0, 0.0, 0.0]
+        db.execute("INSERT INTO legacy_vectors(rowid, emb) VALUES (?, ?)", [i, _f32(vec)])
+
+    db.commit()
+
+    # Verify
+    count = db.execute("SELECT count(*) FROM legacy_vectors").fetchone()[0]
+    print(f"Inserted {count} rows")
+
+    # Test KNN works
+    query = _f32([1.0, 0.0, 0.0, 0.0])
+    rows = db.execute(
+        "SELECT rowid, distance FROM legacy_vectors WHERE emb MATCH ? AND k = 5",
+        [query],
+    ).fetchall()
+    print(f"KNN top 5: {[(r[0], round(r[1], 4)) for r in rows]}")
+    assert rows[0][0] == 1  # closest to [1,0,0,0]
+    assert len(rows) == 5
+
+    # Also create a table with name == column name (the conflict case)
+    # This was allowed in old versions — new code must not break on reconnect
+    db.execute("CREATE VIRTUAL TABLE emb USING vec0(emb float[4])")
+    for i in range(1, 11):
+        db.execute("INSERT INTO emb(rowid, emb) VALUES (?, ?)", [i, _f32([float(i), 0, 0, 0])])
+    db.commit()
+
+    count2 = db.execute("SELECT count(*) FROM emb").fetchone()[0]
+    print(f"Table 'emb' with column 'emb': {count2} rows (name conflict case)")
+
+    db.close()
+    print(f"\nGenerated: {DB_PATH}")
+
+
+if __name__ == "__main__":
+    main()