Add comprehensive fuzz testing infrastructure with 6 new targets

- Fix numpy.c: tautology bug (|| → &&), infinite loop, and missing
  sqlite3_vec_numpy_init call
- Replace tests/fuzz/Makefile: auto-detect clang, add UBSAN, macOS
  ld_classic workaround, generic build rules for all 10 targets
- Add 6 new fuzz targets: shadow-corrupt (corrupted shadow tables),
  vec0-operations (INSERT/DELETE/query sequences), scalar-functions
  (all 18 SQL scalar functions), vec0-create-full (CREATE + lifecycle),
  metadata-columns (metadata/auxiliary columns), vec-each (vec_each TVF)
- Add seed corpora for shadow-corrupt, vec0-operations, exec, and json
- Add fuzz-build/fuzz-quick/fuzz-long targets to root Makefile

All 10 targets verified building and running on macOS ARM (Apple Silicon).

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