Add UBSAN findings TODO and improve vec-mismatch fuzzer

Document three classes of undefined behavior found by UBSAN:
function pointer type mismatches, misaligned f32 reads, and
float-to-integer overflow in vec_quantize_int8.

Improve vec-mismatch fuzzer to cover all error-path cleanup patterns:
type mismatches, dimension mismatches, single-arg functions, and
both text and blob inputs.

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