Preserve VLA source capacity for unequal-allocator moves

[thirtytwobits]

Allocator-extended move construction with unequal allocators currently frees the source buffer after relocating its elements. Retain that allocation and capacity while emptying the source, matching unequal-allocator move assignment. This permits source reuse without another allocation; callers can explicitly release storage with shrink_to_fit().

Equal-allocator moves still transfer storage. Destination allocation sizing and the existing move-assignment policy of releasing an insufficient destination buffer before replacement are preserved. Document the successful-move contract and the separate exception guarantees, and remove no-op allocator assignment calls.

Regression coverage checks ordinary elements, packed bool, empty reserved sources, both move-assignment capacity branches, source reuse and explicit release, copy fallback, move-only elements, allocator destruction hooks, and successful retry after allocation failure. The new constructor-retention tests failed against the original implementation before the fix.

Validation:

• All 10 VLA suites passed with GCC C++14 and Clang C++17 in Debug and ReleaseEP.
• All 215 cases in the move-capacity, exception-safety, and copy/move suites passed with Clang C++17 ASan/UBSan and leak detection.
• Prescribed Toolshed clang-format checks and git diff --check passed.
• API documentation built successfully; 13 warnings remain in unchanged type_traits_ext.hpp and unbounded_variant.hpp.

Fixes #211.