Add bounded HAMT node map mutations

[helly25]

Add the general address-stable HAMT map surface over the shared node machinery. Persistent snapshots share immutable keys and topology, while transient edits detach shared mapped payloads before exposing mutation.

AG;DR

• Add HamtNodeMap<Key, Mapped, Hash, Equal, Options, Source> with persistent insertion and erasure plus a move-only transient editor.
• Keep keys const and store entries in separately owned payloads so surviving entries retain stable addresses across structural mutation.
• Detach shared payloads before mutable at, find, or iteration exposes mapped values; unchanged snapshots remain logically independent.
• Support heterogeneous lookup, full-hash collisions, fragment widths from four through seven bits, bounded sources, maximum-size limits, and recoverable allocation failure.
• Support nothrow move-only mapped values for persistent insertion without multiplying the core HAMT routing implementation.
• Clone topology and payloads transactionally into alternate source domains, preserving the input on failed consuming clones.
• Reject shared-node reference-count overflow instead of allowing ownership counts to wrap.
• Exercise persistent and transient mutation when shared topology or payload allocation is exhausted, preserving every published value.
• Centralize required-access null checks so persistent and transient at retain the same terminating contract without duplicate invariant branches.
• Consolidate invariant-only mutable iterator extraction so recoverable allocation failures remain explicit without duplicating impossible error branches.
• Keep public fallible factories consistent with STL-style container naming through try_create.
• Document iterator/reference lifetime, nothrow constraints, synchronization, source-domain behavior, and the current whole-tree mutable-preparation tradeoff.
• Test persistence, transient mutation, collisions, address stability, move-only values, allocation rollback, child-empty erasure, cloning, size limits, heterogeneous lookup, and all supported fragment widths.

Validation:

• bazel --output_user_root=/private/tmp/mbo-bazel-node-map test --disk_cache=/private/tmp/mbo-bazel-disk-cache //...
• Focused native-Clang LCOV analysis covers every recoverable path; the remaining paths are hard-failure contracts for system allocation or invalid required access.
• Focused pre-commit checks for this edge, with local clang-tidy skipped because CI owns clang-tidy validation.
• Focused public map/set suites pass after the try_create rename.