chore(render): split markdown renderer into modules

[Wpelletier01]

What this PR does

Keep the markdown renderer easier to maintain by separating each output section into focused modules without changing the public rendering API.

The existing renderer tests were moved alongside their section modules so each file stays smaller and easier to review, including respecting the 25 KB size limit.

Test coverage

• No new tests (explain why below)

No new behavior was added. Existing markdown renderer unit coverage was preserved and split into the relevant module files with the implementation.

Verification gates

• cargo fmt --all --check clean
• cargo clippy --all-targets --all-features -- -D warnings clean
• cargo test --release clean
• bash examples/run-all.sh clean (if behavior or rendered-output changes touched the example fixtures)
• mdbook build docs clean (if any docs/src/**.md changed)

bash examples/run-all.sh is not clean because the pinned example outputs are not up to date. The current renderer emits the Maintainer set changed summary row and detail section, but examples/*/expected-output.md still reflects the older output shape.

Linked issues

#30

[Metbcy]

Code Review Summary

Verdict: Approved

No blocking findings.

What I checked

• Renderer split from the old monolithic src/render/markdown.rs into focused modules.
• Public API compatibility for render, render_with_options, Options, and Platform.
• Section ordering and short-circuit behavior for empty diffs and summary-only mode.
• Deterministic rendering concerns, especially sorted vulnerability and plugin output.
• Merge-ref validation against current base.

Verification

• cargo fmt --all --check clean
• cargo clippy --all-targets --all-features -- -D warnings clean
• cargo test --release clean: 391 unit tests, 36 CLI tests, 9 integration tests, 7 real-world tests, 1 doctest

Looks like a clean maintainability refactor with behavior preserved.

[Metbcy]

Thank you for your collaboration @Wpelletier01!