Add vec-mismatch fuzz target that catches aCleanup(a) bug in ensure_vector_match

Targeted fuzzer for two-argument vector functions (vec_distance_*,
vec_add, vec_sub) that binds a valid JSON vector as arg1 and fuzz
data as arg2. This exercises the error path in ensure_vector_match()
where the first vector parses successfully (with sqlite3_free cleanup)
but the second fails, triggering the buggy aCleanup(a) call on line
1031 of sqlite-vec.c (should be aCleanup(*a)).

The fuzzer catches this immediately — ASAN reports "bad-free" when
sqlite3_free is called on a stack address.

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