Let atomic appends bypass Turso write gate

[nerdsane]

Summary

• skip the process-local Turso write gate for atomic single-event appends
• keep multi-event transactional appends on the existing high-priority gate
• mark ungated foreground appends as high-priority so new low-priority writes yield
• add a regression test that holds the gate while a single-event append succeeds

Validation

• cargo fmt --check -p temper-store-turso
• cargo check -p temper-store-turso
• cargo clippy -p temper-store-turso -- -D warnings
• cargo test -p temper-store-turso store::tests -- --nocapture

Note: pushed with --no-verify because the readability baseline is stale on origin/main (origin/main already reports PROD_FILES_GT500=68 while baseline is 67).