Add borrowed forward iteration over HAMT snapshots

[helly25]

Add forward traversal over HAMT bitmap nodes and collision entries as the next step toward general map/set containers. Iterator copies carry independent traversal positions without allocating traversal storage.

AG;DR

Adds the shared-node iterator library, fixture-based tests, Bazel targets, and lifetime documentation. Traversal uses an inline bounded frame stack derived from hash width and fragment width. Entries remain borrowed: the snapshot root and allocation source must outlive iteration. End iterators compare equal; non-end positions include root identity. Public container range identity is a later layer.

Tests cover empty roots, bitmap data, children, collisions, independent iterator copies, and forward-iterator concepts. Initial validation passed 163 tests with one exception-only target skipped, measured focused LLVM coverage of 39/39 lines and 12/12 branches, and passed focused ASAN. These historical results are not substituted for current-head validation.

GCC 14 required explicit HamtInsertResult types for all four optional fallbacks. The subsequent CI clang-tidy diagnostics are corrected by explicitly parenthesizing the depth division and making three unchanged test iterators const. Focused tests and focused pre-commit pass on head 14d49ff, with local clang-tidy and trunk formatting disabled as requested. Full current-head local validation and replacement remote CI are in progress. macOS validation uses native Apple ld for SDK compatibility with the hermetic compiler.

C++20 compatible. No coverage scope, exclusion, threshold, or baseline changes. Draft implementation step; no merges or performance conclusions.