fix: carry TOC inserts across persisted segments - #62

Summary

• carry table-of-contents inserts through existing segments after reload resets the cursor
• replace existing identities without duplicate cross-segment mappings or size inflation
• extend the variable-string persistence regression across a close/reopen boundary

Incident

AgencyZero 0.3.30 stopped persistence with unsized index split identity did not become canonical (page=PageId(218)). The beta.9 invariant correctly surfaced the failure. IndexTableOfContents::insert removed a new identity from a full earlier segment, advanced to the existing tail, and returned without inserting it.

The focused regression fails before this fix with None for the inserted PageId and passes after it.

Validation

• 184 library tests passed
• 522 integration tests passed, 4 ignored
• 2 doctests passed
• cargo clippy --all-targets --all-features -- -D warnings
• close/reopen plus 1,000 varying-size string upserts passed