[#456] Add namespaced pool storage keys with TTL management

[arisu6804]

Summary

This PR implements namespaced pair storage and TTL management for #456.

• adds typed pair namespaces and slot descriptors covering all existing pair fields;
• maps every typed slot directly to the deployed DataKey variant, preserving XDR/storage layout;
• centralizes persistent TTL thresholds and extension targets;
• renews existing slots on access and after writes without creating orphan keys;
• keeps batch operations within a bounded gas budget by using per-key bumps in hot loops;
• preserves config/history clearing semantics across unregister and explicit metric purge;
• adds append-only layout ordinals, compatibility tests, and storage/operations documentation.

The change set contains 587 added lines of implementation, tests, and documentation.

Validation

• git diff --cached --check
• cargo test --lib pool_storage::tests — 14 passed
• cargo test --lib test_batch::test_register_pairs_exact_max_batch_size_succeeds — passed

Closes #456