Expose cross-source HAMT map and set clones

[helly25]

Flat HAMT maps and sets now expose explicit cross-source cloning, letting callers move a complete persistent value into the allocation domain whose guarantees they need. Ordinary copies remain cheap shared snapshots; changing domains is visible, fallible, and deep.

AG;DR

• adds source-independent identity and pair-key extractors shared by map and set cores;
• exposes try_clone_to<OtherSource>(...) on persistent flat maps and sets;
• exposes consuming transient clone overloads that return persistent destination containers;
• preserves hash/equality state, keys, mapped values, and source snapshots for const-lvalue cloning;
• consumes an rvalue source only after a successful deep clone and preserves it after failure;
• makes destination source type and constructor arguments explicit, including bounded inline sources;
• documents ownership of source objects versus external buffers or PMR resources borrowed by them;
• validates public destination types, independent addresses, successful source reuse, transient cloning, and bounded failure;
• validated locally with the focused key-extractor, flat-map, and flat-set tests; the complete GCC/Clang/sanitizer/coverage matrix will run in this PR's own context.

This is a draft stacked on the owned-domain clone layer in #479. It must remain unmerged until the implementation stack is reviewed and merge-ready as a whole.