Add the shared HAMT bitmap core

[helly25]

Introduce the constexpr occupancy-and-rank primitive shared by the planned HAMT map/set and flat/node layouts. It supports every 4- through 7-bit fragment width selected for later benchmarking, including the two-word 7-bit representation.

AG;DR

• adds one shared internal bitmap implementation rather than multiplying map/set or flat/node cores
• supports membership, dense child rank, insertion, erasure, and population count
• handles 16-, 32-, 64-, and 128-slot nodes
• exhaustively tests every slot at every candidate fragment width
• verifies constant-evaluation support
• follows the internal PascalCase API convention and uses bounds-checked array access required by repository clang-tidy policy

Validation:

• bazel test //mbo/container:hamt_bitmap_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 #445 and must not merge before its predecessors. It is an internal complete primitive, not a prematurely exposed partial HAMT container.