chore: fold #663 into 0.18.9, revert premature 0.18.10 bump

[iheitlager]

Summary

• perf: build IdxInsert keys from live registers instead of re-seeking (#663) #668 (fixing perf: INSERT/UPDATE codegen re-seeks table cursor to rebuild index keys instead of reusing registers #663) bumped to 0.18.10, but 0.18.9 wasn't tagged yet at that point — no reason to burn a separate version for it.
• Folds perf: INSERT/UPDATE codegen re-seeks table cursor to rebuild index keys instead of reusing registers #663's changelog entry back into the 0.18.9 section (alongside perf: SorterInsert re-decodes GROUP BY sort-key from the record blob it just built #660/perf: IndexCursor::seek does linear scan instead of binary-search tree descent #661) and reverts Cargo.toml/README.md back to 0.18.9.

Test plan

• cargo build — Cargo.lock regenerated at 0.18.9
• cargo clippy/cargo fmt --check (pre-push hook) — clean

spend: trivial