Add unicode pass/fail icons to markdown rule summary

[jesse-black]

Motivation

• Improve the readability of the generated Markdown summary by prefixing rule statuses with clear unicode indicators (✅ / ❌) so CI/PR viewers can scan pass/fail rows more quickly.

Description

• Render rule statuses in the Diff Coverage table as ✅PASS or ❌FAIL instead of plain PASS/FAIL by updating src/render/markdown.rs.
• Update existing unit assertions in src/render/markdown.rs to expect the ❌FAIL text for failing rules and add a new unit test that verifies both ✅PASS and ❌FAIL rows are produced.
• Update the CLI markdown integration expectation in tests/cli_interface.rs to match the new ✅PASS output format.

Testing

• Ran cargo test render::markdown::tests --lib and all renderer tests passed (including the new renders_rule_status_with_unicode_icons test).
• Ran full validation with cargo xtask validate, which completed successfully after recording a base ref with cargo run --bin covgate -- record-base to satisfy the validation check-run; validate reported all checks passed.
• Ran cargo fmt as part of validation and formatting checks passed.

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Codex Task