Refactor temper-spec for guideline compliance

[nerdsane]

Summary

• split the remaining temper-spec parser and extractor monoliths into focused modules
• move bulky inline tests into dedicated test files and tighten a weak integration default test
• keep temper-spec green under fmt, tests, and clippy while bringing files/functions under the guideline thresholds

Verification

• cargo fmt --all
• CARGO_TARGET_DIR=target/codex-temper-spec cargo test -p temper-spec
• CARGO_TARGET_DIR=target/codex-temper-spec cargo clippy -p temper-spec --all-targets -- -D warnings