feat: batch source span resolution

[Iktahana]

Summary

• add Rust-backed resolveMdiSourceSpans(source, spans) with one parse/projection per batch
• keep singular resolution as a convenience wrapper with identical mapping semantics
• cover repeated identical text and every searchable block family, including footnotes
• expose the batched JSON/WASM boundary in Node and browsers and document it in English, Japanese, and Traditional Chinese

Why

PR #75 established the source-span inverse mapping contract, but repeated singular calls reparsed a full document. This follow-up provides the batch path needed by diagnostics and decoration consumers while preserving input order and all validation/coverage/relation rules.

Validation

• cargo fmt --check
• cargo clippy --all-targets --all-features -- -D warnings
• cargo test
• Rust llvm-cov thresholds: lines 96.94%, functions 93.33%, regions 95.54%
• pnpm typecheck
• pnpm build (including docs)
• pnpm --filter @illusions-lab/mdi test (83 tests)
• pnpm test:browser (packed Chromium, Firefox, WebKit)
• publishable-package contract tests
• pnpm test:coverage

Release safety

The earlier main CI run was cancelled after the performance gap was identified, and its release workflow skipped. This PR must pass PR and post-merge main CI before automatic publication.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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