Add focused build-review renderer tests

[coreytshaffer]

Summary

• Adds one focused test file for the public render_markdown and render_html build-review report APIs.
• Covers deterministic repeated rendering, adversarial HTML escaping, Markdown fence neutralization, em-dash rendering for unknown counts, empty-collection fallbacks, and pending/completed decision views.
• Makes no production behavior change.

Opus 5 coding trial

This change originated as a bounded Opus 5 coding trial and is presented for review on its technical merits:

• one new test file;
• ten focused tests;
• production code untouched;
• configuration, dependencies, CLI, persistence, authorization, routing, execution, and documentation untouched.

The trial succeeded on coding, testing, scope retention, and self-review. A persistent minor limitation remains in exact output-format compliance, which does not affect this diff.

Validation

• Focused: 10 passed in 0.13s
• Full suite: 1249 passed, 5 skipped, 1 xfailed in 106.19s
• The existing CR-YK-001 concurrency xfail is unchanged.
• git diff --cached --check passed.
• Staged scope contained exactly tests/test_build_review_report.py.

Deferred observation

Review identified three raw HTML interpolations in the production renderer: files_changed, finding severity in a class attribute, and completed decision status in a class attribute. Ordinary packet construction constrains these values, so this is a small defense-in-depth hardening candidate rather than an emergency vulnerability.

That production hardening is intentionally excluded from this test-only PR and requires a separate bounded change.

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