refactor(enrich/maintainer): split into submodule directory (#57 2/3)

[Metbcy]

Part 2 of 3 for #57 (module size budget round 2). Follows the same pattern as #62 (run.rs split).

What

src/enrich/maintainer.rs (1284 LOC) split into src/enrich/maintainer/ (8 files):

• mod.rs (79): header doc, module decls, pub use re-exports
• types.rs (74): Host, MaintainerAgeFinding, MaintainerInfo, LookupError, response structs, constants
• pipeline.rs (206): enrich, enrich_with, enrich_with_hosts
• github.rs (118): lookup_github_repo, github_get
• gitlab.rs (129): lookup_gitlab_repo, gitlab_get
• codeberg.rs (19): lookup_codeberg_repo
• parsing.rs (286): URL parsers, ISO-8601, percent_encode, JSON extractors, Link header
• tests.rs (421): full #[cfg(test)] block

Biggest non-test file: parsing.rs at 286 LOC. Well under the 600 budget.

Behavior change

None. Pure mechanical cut/paste. Public API preserved via pub use in mod.rs (verified pub(crate) helpers like parse_github_repo and iso8601_to_unix_seconds still resolve at their old paths).

Verification

• cargo test --lib: 429 passed, 0 failed
• cargo clippy --all-targets -- -D warnings: clean
• cargo test --lib enrich::maintainer: 43 passed

Follow-up

3/3 of #57: split src/render/sarif.rs (1205 LOC). Coming next.

[github-actions]

Coverage report

Line coverage: 84.2% (9585 / 11384 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.}