test: add 56 unit tests for semantic, slab_alloc, and ast_profile modules

[RollerCoaster1899]

Summary

Adds 56 unit tests across 3 new test files for previously untested modules:

• tests/test_semantic.c (32 tests): cbm_sem_tokenize (camel/snake/dot/abbrev), cbm_sem_cosine (identical/orthogonal/zero/negative/null), cbm_sem_normalize, cbm_sem_vec_add_scaled, cbm_sem_random_index (determinism), cbm_sem_proximity, cbm_sem_diffuse, corpus lifecycle, IDF, config
• tests/test_slab_alloc.c (14 tests): Slab malloc/free (≤64B and >64B), calloc zeroing/overflow, realloc (shrink/promote/same-size), free-list cycling, pointer uniqueness, thread reset
• tests/test_ast_profile.c (10 tests): to_str/from_str round-trip (25 fields), null/empty/invalid inputs, to_vector range/saturation

Verification

./build/c/test-runner semantic ast_profile slab_alloc

All 56 pass with ASan+UBSan.

Files modified

• Makefile.cbm: Register new test sources in ALL_TEST_SRCS
• tests/test_main.c: Register suite_semantic, suite_ast_profile, suite_slab_alloc

[DeusData]

Huge thanks for opening this PR and for the work you put into it.

The maintainer shop is currently full, so this may sit for a bit before it gets a proper review. We will come back to this as soon as possible with real feedback; I wanted to make sure it did not sit unacknowledged in the meantime.

[DeusData]

Thanks for adding coverage. Before review can continue, please add a DCO Signed-off-by trailer to the non-merge commit and link the issue or coverage goal this addresses. Since these tests are added to ALL_TEST_SRCS, please also include the full test command/output.

[DeusData]

Thank you — these are genuine, non-vacuous guards (exact-value assertions on the semantic proximity/cosine math, slab calloc-overflow + realloc-promotion, and the 25-field ast_profile roundtrip), not coverage padding, and they're black-box on the public slab API. That last part is timely: there's an in-flight slab-allocator redesign (cross-thread frees / #852), and this suite is exactly the independent regression net it needs. Merged as a4e1f21. Appreciated!