fix: preserve footnotes in canonical serialization

[Iktahana]

Closes #80.

Summary

• serialize GFM footnote definitions from Rust-owned mdast, including multi-paragraph and nested block content
• serialize reference link definitions with labels, URLs, and optional titles
• cover numeric and named footnotes, Unicode/ZWJ, nested lists, reference definitions, and canonical idempotence in Rust and Node

Validation

• cargo fmt --check
• cargo clippy -- -D warnings
• cargo test (119 passed, 5 ignored across Rust suites)
• pnpm build
• pnpm --filter @illusions-lab/mdi test (85 passed)
• pnpm typecheck

[codecov]

Codecov Report

❌ Patch coverage is 93.67089% with 5 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
mdi-core/src/lib.rs	93.67%	5 Missing ⚠️

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