Add note export profile

[Iktahana]

What changed

• add a Rust-backed note text export profile with native ruby output
• expose note through the JavaScript, CLI, Python, and Android text-format APIs
• include note in txt-all packaged-consumer output
• add focused Rust, JavaScript, CLI, Python, JNI, and Android contract coverage
• document official note editor shortcuts, readable fallbacks, and UI-only capabilities in English, Japanese, and Traditional Chinese
• make generated EPUB 3.3 documents conformant when chapters have no heading or source images are not packaged

Why

MDI did not have a text profile tailored to note. The profile converts representable document structure to note-oriented editor input while preserving readable content when note has no equivalent text syntax. It is not a WXR or MT import archive.

Validation

• Rust: format, 74 tests, and clippy with warnings denied
• JavaScript: build 12/12, typecheck 19/19, test 23/23
• isolated npm package consumer: 3/3, including six txt-all outputs
• Python: rebuilt abi3 wheel, 20/20 tests, isolated package smoke
• EPUBCheck 5.3.0: 0 errors and 0 warnings on Kitchen Sink EPUB
• DOCX: Open XML Validator passed; LibreOffice imported and rendered 17 pages
• PDF: PDF.js suite passed; Kitchen Sink PDF parsed, extracted, rendered, and visually inspected
• documentation build: 347 pages

Swift release files remain intentionally outside this PR; the CI Swift job validates repository compatibility.

[codecov]

Codecov Report

❌ Patch coverage is 95.78544% with 22 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
mdi-core/src/lib.rs	95.76%	22 Missing ⚠️

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