fix: keep TOC reload insertion on the fast path

[pathscale]

Follow-up to #62.

• removes the unconditional duplicate scan from every TOC insertion
• removes duplicate CreateNode insertions at their sized and unsized callers
• probes segment capacity without mutating persisted size accounting
• returns an explicit error for a truncated loaded segment chain
• strengthens reload and cursor-placement regression coverage

Validation:

• cargo test --workspace --all-targets
• cargo clippy --workspace --all-targets --all-features -- -D warnings
• repeated varying string upsert reload regression

Resolves the performance and size-accounting findings raised in review of #62.