feat: resolve MDI source spans

[Iktahana]

Summary

• add Rust-owned resolve_mdi_source_span resolution from half-open UTF-8 source spans to canonical block/grapheme ranges
• expose typed Node and browser WASM APIs with explicit coverage, relation, channel, ordering, and validation semantics
• retain comprehensive Rust, TypeScript, projection-version, round-trip, and packed-browser contract tests
• document boundary, delimiter, ruby, synthetic/unmapped, multi-block, and non-bijective behavior in English, Japanese, and Traditional Chinese

Related issue

Closes #74.

Validation

• cargo fmt --check
• cargo clippy -- -D warnings
• cargo test
• cargo llvm-cov --locked --fail-under-lines 90 --fail-under-functions 90 --fail-under-regions 90 --summary-only
  • lines 96.88%, functions 93.12%, regions 95.52%
• pnpm install --frozen-lockfile
• pnpm typecheck
• pnpm build
• pnpm test:browser (Chromium, Firefox, WebKit)
• node --test nodejs/scripts/pack-publishable-package.test.mjs
• pnpm test:coverage

Checklist

• This PR is focused and does not include unrelated changes.
• I added or updated tests for behavior changes.
• I updated the specification and/or documentation when public behavior changed.
• I ran the relevant checks and recorded them above.
• The repository's release workflow computes patch releases from the merged diff; no Changeset file is used.
• I have not added secrets, credentials, or sensitive data.

Notes for reviewers

The projection wire shape and MDI_TEXT_PROJECTION_VERSION remain 1.0; this adds a separate result boundary without changing the existing projection envelope. Package versions are intentionally left to the repository's post-merge release workflows.

[codecov]

Codecov Report

❌ Patch coverage is 96.64804% with 6 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
mdi-core/src/text_projection.rs	96.64%	6 Missing ⚠️

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