Expose StringInterner parent growth and root identity

[helly25]

StringInterner children can now identify their topmost root and detect whether their direct parent has grown since the child captured its visible prefix. This supports ownership, lifecycle, and cache-management decisions without changing snapshot visibility or adding hot-path bookkeeping.

AG;DR

• Add root() to follow borrowed parent links to the topmost interner, including through parents whose captured prefix is empty.
• Add parent_has_grown() to compare the direct parent's current visible size with first_local_id().
• Keep the frozen-prefix contract unchanged: later parent or ancestor strings do not become visible to an existing child.
• Avoid a stored root pointer, revision counter, or separate parent-presence state; the existing parent link and captured boundary are sufficient.
• Document direct-parent-only semantics, synchronization, and ancestor-lifetime requirements.
• Test root identity, direct versus distant ancestor growth, duplicate insertion, and unchanged child lookup visibility.

Validation:

• bazel test //mbo/strings:string_interner_test
• Focused pre-commit checks pass with local clang-tidy and trunk formatting intentionally skipped; CI remains authoritative for clang-tidy.

Known limitation: these borrowed relationships continue to require externally managed ancestor lifetimes and synchronization, matching the existing StringInterner contract.