Add transactional Arena string storage

[helly25]

Add an Arena-backed string ownership adapter that copies bytes once, returns stable std::string_views, and can roll back an uncommitted copy when another interner component fails. This supplies the byte-storage half of bounded, allocation-controlled string interning without coupling storage to indexing or ID assignment.

AG;DR

• Add ArenaStringStorage, parameterized by an Arena-compatible type.
• Return only owned std::string_views and preserve embedded NUL bytes.
• Make empty-string storage allocation-free.
• Surface bounded exhaustion as std::nullopt.
• Expose Arena checkpoints and rewind so compound interner insertion can transactionally discard unpublished bytes.
• Keep the adapter non-copyable and non-movable because published views depend on its stable ownership domain.
• Document the separation between byte ownership, indexing, and dense-ID assignment.
• Test ownership, embedded NULs, rollback/reuse, empty strings, and bounded exhaustion.

Validation:

• bazel --output_user_root=/private/tmp/mbo-arena-string-storage-bazel test //mbo/strings:arena_string_storage_test

This PR is stacked on #494.