Add HAMT hash-path traversal

[helly25]

Add the shared constexpr primitive that splits 32- and 64-bit hashes into HAMT traversal fragments. The same implementation covers every 4- through 7-bit candidate width without coupling dense IDs to hash width.

AG;DR

• adds unsigned 32- and 64-bit hash-path traversal
• computes the required trie depth at compile time
• handles partial final fragments without reading beyond the hash width
• reconstructs representative hashes in tests for every candidate fragment width
• validates constexpr use and the non-divisible width cases
• follows the internal PascalCase naming policy for hash-path operations

Validation:

• bazel test //mbo/container:hamt_hash_path_test
• focused pre-commit passed with local clang-tidy and the sandbox-incompatible trunk wrapper explicitly skipped; clang-format passed

This is a chained draft based on PR #446 and must not merge before its predecessors.