coproc/vecaccess/mxaccess/lxaccess: extend test suite and refactor

[zbelinsk]

Add comprehensive unit tests for mxaccess and lxaccess (previously untested), and replace the vecaccess smoke test with a full path-testing suite. All three tests share the same structure: unit_init with state verification, acquire/release/reuse bounded by a hardware-queried limit, a limit+1 blocking acquire via a second thread, double-release rejection, and a legacy-wrapper check. The tests derive the context limit by calling unit_init first (which latches the hardware style), then using h2_coproc_count() sign to mirror the impl's semaphore init logic for both oldstyle and newstyle hardware.

Refactors in the implementation:

• h2_coproc_init: fix one-shot regression (re-report OLDSTYLE on every call once latched; set init_done only after the full unit loop succeeds); add h2_coproc_init_result_t enum replacing magic 0/1/-1
• h2_coproc_set: replace per-call bit-scan with O(1) bitpos table lookup built at init time; remove redundant Q6_R_popcount_P call (use pre-computed counts[] array); remove dead bits local
• h2_coproc: extract coproc_grow() helper, eliminating repeated realloc-assign-null-check pattern across init_entry_contexts and h2_coproc_init
• h2_vecaccess/mxaccess/lxaccess release: add double-release guard (check active bit before releasing); fix idx passed to h2_coproc_set (was hardcoded 0); remove redundant h2_hwconfig_extbits call in vecaccess release
• h2_vecaccess new-style branch: add missing HVX_64 case; align SILVER/SILVER_MAX sem init to use h2_coproc_count (was h2_info)
• h2_vecaccess.h: add H2_VECACCESS_MAX_VLENGTH_BYTES replacing bare 128