fix: distinguish escaped literal syntax at parse time

[Iktahana]

Summary

• mark escaped Markdown/MDI text in the Rust mdast bridge without changing normal canonical serialization
• include the leading escape bytes in literal and semantic source spans
• retain table and footnote canonicalization while exposing literal intent to editor adapters

Verification

• cargo test --manifest-path mdi-core/Cargo.toml
• cargo clippy --manifest-path mdi-core/Cargo.toml --all-targets --all-features -- -D warnings
• cargo fmt --manifest-path mdi-core/Cargo.toml --check
• downstream milkdown-plugin-mdi full suite: 180 passed, 2 skipped

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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