Validate PQ header invariants on open

Reject unsupported or inconsistent PQ parameters in open() before
constructing ProductQuantizer:
- ksub must be 16 (4-bit) or 256 (8-bit)
- d must equal m * dsub
- 4-bit PQ (ksub=16) requires even m

This prevents malformed headers from reaching search code with
nonsensical parameters. Also fix test_huge_pq_section_size_returns_error
to use valid ksub=256 so it tests the section-size cap path instead of
being caught earlier by the new ksub check.

- Add test_unsupported_ksub_returns_error
- Add test_d_not_equal_m_times_dsub_returns_error

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