Fix determine_update_operation to correctly detect INSERT with explicit rowid

[russellromney]

Summary

Fixes a bug in determine_update_operation that caused INSERT operations with explicit rowid to panic.

The existing code checked argv[1] == NULL to detect INSERT operations, but according to SQLite xUpdate documentation, argv[0] == NULL indicates INSERT:

If argc is greater than 1 then an insert occurs when argv[0] is an SQL NULL.

The Bug

When inserting with an explicit rowid:

INSERT INTO vtab(rowid, col) VALUES (1, 'value')

1. SQLite calls xUpdate with argv[0]=NULL (indicating INSERT) and argv[1]=1 (the rowid)
2. The code incorrectly checked argv[1] for NULL
3. Since argv[1]=1 (not NULL), it fell through to the unimplemented todo!() branch

The Fix

• Check argv[0] for NULL to detect INSERT (matches SQLite docs)
• Compare argv[0]/argv[1] values (not pointers) for UPDATE detection
• Remove todo!() by treating rowid-change UPDATE as regular UPDATE

Testing

Tested with sqlite-tantivy, a Tantivy-powered FTS virtual table that uses explicit rowids:

CREATE VIRTUAL TABLE articles USING tantivy(title TEXT, body TEXT);
INSERT INTO articles(rowid, title, body) VALUES (1, 'Hello', 'World');
-- Previously panicked, now works correctly

All 31 tests pass (25 unit + 6 integration).