vec0 UPDATE SET rowid reports success but silently leaves the rowid unchanged #332

Description

On a vec0 virtual table, UPDATE ... SET rowid = <new> reports rowcount = 1 (one row affected) but silently leaves the rowid unchanged. The caller is told the update succeeded; the table did not change.

Environment

• sqlite-vec 0.1.9 (pip loadable extension), Python 3.12 sqlite3, Windows x64

Steps to reproduce

db.execute("CREATE VIRTUAL TABLE v USING vec0(a float[2])")
db.execute("INSERT INTO v(rowid, a) VALUES (1, ?)", ...); db.execute("INSERT INTO v(rowid, a) VALUES (2, ?)", ...)
cur = db.execute("UPDATE v SET rowid = 3 WHERE rowid = 2")
print(cur.rowcount)                                   # 1  <- claims success
print(db.execute("SELECT rowid FROM v").fetchall())   # [(1,), (2,)]  <- unchanged

Full script attached (reproduce.py).

Expected behavior

Either the rowid update is applied (rowid 2 becomes 3), or the statement fails with a clear error ("rowid updates are not supported on vec0 tables"). Silently reporting success while doing nothing violates SQLite's statement semantics.

Actual behavior

rowcount=1, no change, no error.

Additional notes

• UPDATE of the vector column itself works correctly (verified: stored vector, distances and metadata all update consistently) — only the rowid target is affected.
• Plain-table control: UPDATE t SET rowid_ = 3 WHERE rowid_ = 2 applies normally.