Add HAMT node occupancy indexing

[helly25]

Add the shared CHAMP-style node occupancy primitive for HAMT map/set and flat/node implementations. Separate data and child bitmaps map logical hash fragments to dense storage indices while enforcing that a slot cannot hold both forms.

AG;DR

• models empty, inline-data, and child-node slots
• computes independent dense data and child indices through bitmap rank
• supports insertion, erasure, promotion, and demotion without layout duplication
• supports all benchmark-candidate fragment widths from 4 through 7 bits
• tests disjoint occupancy, rank maintenance, transitions, and constexpr use
• follows the internal PascalCase naming policy throughout the node-index API

Validation:

• bazel test //mbo/container:hamt_node_index_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 #447 and must not merge before its predecessors.