Reuse HAMT key extractors for separately owned node payloads

[helly25]

Address-stable HAMT containers can now reuse the existing map and set key-extraction machinery while their payloads live behind ownership handles. Keys remain borrowed directly from the stored payload, so lookup adds no copies or allocations.

AG;DR

• Add HamtNodeKeyOf<KeyOf> to compose an existing HAMT key extractor with a separately owned payload handle.
• Dereference the handle once and invoke the configured set or map extractor on the stored value.
• Preserve reference identity for both set keys and immutable map keys.
• Constrain the adapter to nonthrowing handle access and nonthrowing key extraction, matching HAMT hot-path requirements.
• Verify constexpr set projection, map-key projection, rejected throwing extractors, and integration with shared HAMT storage.
• Exercise address stability across persistent mutation through the shared core.
• Document that handles must be nonempty and returned references borrow the payload lifetime.

Validation:

• bazel --output_user_root=/private/tmp/mbo-bazel-rollback test --disk_cache=/private/tmp/mbo-bazel-disk-cache //...
• focused pre-commit checks for this edge, with local clang-tidy intentionally skipped because CI owns it