fix(core): conform platform exports to official contracts

[Iktahana]

Summary

• add contract tests sourced from the official Narou, Kakuyomu, and Aozora Bunko documentation
• enforce documented ruby limits, delimiter escaping, emphasis exclusions, Aozora annotations, heading hierarchy, CRLF, and Shift_JIS behavior
• bump the Rust crate to 2.0.3 and document the platform contracts

Verification

• cargo test / fmt / clippy / package
• Rust coverage gates (90% lines/functions/regions)
• pnpm typecheck / build / test / test:coverage
• publishable npm tarball consumer install test
• dev CI, Android, and Python workflows

[codecov]

Codecov Report

❌ Patch coverage is 98.01587% with 5 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
mdi-core/src/lib.rs	98.01%	5 Missing ⚠️

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