Constrain HAMT node-map payload detachment APIs

[helly25]

Make the public HAMT node-map API advertise its real payload requirements. Operations that detach or clone shared payloads now participate only when the complete key/value entry is nothrow-copy-constructible, preventing invalid templates from surviving until deep implementation errors.

AG;DR

• constrain persistent and transient try_update on nothrow copy construction of the full value_type;
• apply the same requirement to lvalue and consuming try_clone_to overloads;
• retain lookup and mutation support for move-only mapped values where no payload detachment is required;
• verify the public overload surface with compile-time availability checks for copyable and move-only payload combinations;
• keep allocation and mutation failure behavior unchanged for supported operations.

Validation:

• bazel test //mbo/container:hamt_node_map_test
• focused pre-commit hooks on both changed files, with local clang-tidy intentionally left to CI