Add transactional Arena checkpoints

[helly25]

Add borrowed Arena checkpoints that let multi-part operations reclaim an uncommitted byte tail without releasing reusable capacity. This provides the transactional storage primitive needed by fallible arena-backed string interning.

AG;DR

• Add Arena::Checkpoint, checkpoint(), and rewind().
• Preserve allocations made before a checkpoint while invalidating and making later storage reusable.
• Retain reserved blocks and growth progression so rollback does not turn bounded failure handling into allocation churn.
• Enforce checkpoint ownership through the existing configurable contract mechanism; rewind() is conditionally noexcept when contract violations cannot throw.
• Document lifetime, invalidation, synchronization, and raw-object-destruction constraints in mbo/memory/ARENA.md.
• Test empty, nested, and same-block/cross-block checkpoint rollback and storage reuse.

Validation:

• bazel --output_user_root=/private/tmp/mbo-arena-checkpoint-bazel test //mbo/memory:arena_test

This PR is stacked on #492.