refactor(baseline): split baseline.rs into a submodule directory (#68)

[Metbcy]

Summary

Splits the 1274-LOC src/baseline.rs into a src/baseline/ submodule directory, the next file over the 1000-LOC soft cap after the #57 series (PRs #62-#65). Closes #68.

This is a pure mechanical move: no logic, control flow, or string literal changed. The layout mirrors the reference shape from PR #62 (run/) and #64 (maintainer/).

Layout

File	LOC	Responsibility
mod.rs	302	Crate doc, Baseline / BaselineEntry / ExpiredEntry types and the parsing core (load, from_value, from_value_strict), plus the re-export wiring
apply.rs	49	The suppression pass (apply)
mutate.rs	140	Baseline-file writing (add_suppression, add_suppression_full, AddOutcome)
comment.rs	70	PR/MR comment directive parsing (parse_comment_directive)
tests.rs	741	The #[cfg(test)] mod tests block, moved verbatim

Why

baseline.rs was the largest non-test file over the soft cap. The split keeps each concern in its own file so review attention does not skip the middle of a 1200-line module, and it follows the budget rule in docs/src/architecture.md.

Compatibility

The public surface stays reachable at the same paths (crate::baseline::{Baseline, BaselineEntry, ExpiredEntry, apply, add_suppression, add_suppression_full, parse_comment_directive, AddOutcome} plus the Baseline methods). pub mod baseline; in src/lib.rs is unchanged. doc_kind was widened from private to pub(super) since it is now shared across mutate.rs and the tests. No other visibility changed.

Testing

• cargo build --all-targets --all-features (RUSTFLAGS="-D warnings"): clean
• cargo clippy --all-targets --all-features -- -D warnings: clean
• cargo test --all-features: 441 lib + 36 cli + 9 integration + 7 real_world + 1 doc, all pass
• cargo fmt --all --check: clean

Closes #68.

[github-actions]

Coverage report

Line coverage: 82.9% (8677 / 10470 lines)

Full lcov report available as workflow artifact coverage-lcov: download from this run.

_{v0.9.8 introduces this report; --fail-under-lines will be added once coverage is visible across 2–3 releases.}