feat(core): opt-in preserveWindowsPaths md4c flag - #1105

What/Why?

CommonMark reads every \ before punctuation as an escape, so an agent's message Saved to C:\Users\me\.t3\_build\shot.png renders as C:\Users\me.t3_build\shot.png. Coding-agent output mentions Windows paths all the time, and the dropped separators make them wrong.

This adds an opt-in md4cFlags.preserveWindowsPaths (off by default, so CommonMark output is unchanged). With it on, a backslash escape inside a Windows drive path in prose keeps its backslash:

off: Saved to C:\Users\me.t3_build\shot.png and C:/work.config and C:\folder(draft.
on:  Saved to C:\Users\me\.t3\_build\shot.png and C:/work\.config and C:\folder\(draft.

A path starts at a drive letter that is not preceded by a word character (C:\, C:/, or a bare C: right before the escape) and runs through formatting spans such as C:/Users/**me**\.config. Whitespace, line breaks, code, math and images end it. Past whitespace nothing tells a path from a sentence, so C:\tmp \*x\* still gives *x*. Code spans, link destinations and titles, and image alt text are unchanged.

To tell an escape from the literal character, MD4CParser uses the new ENRMRKD_FLAG_ESCAPEDTEXT / ENRMRKD_TEXT_ESCAPED from software-mansion-labs/md4c#5. That PR needs to merge first. packages/core/cpp/enrmrkd/ here is byte-identical to its head, and I'll re-run scripts/fetch-md4c.sh once it lands. The check scans only text appended since the last escape, so a paragraph with many escapes is not rescanned.

The flag is plumbed like preserveBlankLines: React Native (iOS, Android, web, measurement cache keys), the iOS and Android SDKs, the WASM wrapper (bundle rebuilt), and the docs.

Testing

• New windows-paths.md fixture in the golden AST dump. The flag is on in the extensions-on variants and off in the rest, so the golden records both behaviors. No existing fixture's AST changed. scripts/test-core-parser.sh and scripts/test-web-bundle.mjs pass.
• md4c-flags.test.tsx covers the default and forwarding of the new prop. The React Native package's tsc and Jest suites pass (1,791 tests).
• enriched-markdown-ios builds for the iOS Simulator, and T3 Code's React Native iOS and Android dev clients build with this change. enriched-markdown-android: :parser:assembleDebug, :ui:testDebugUnitTest and ktlintCheck pass.

PR Checklist

• Code compiles and runs on iOS
• Code compiles and runs on Android
• Updated documentation/README if applicable
• Ran example app to verify changes, verified in a downstream app
• E2E tests are passing
• Required regression tests have been added

Model: Claude Opus 5.5. Harness: Claude Code in T3 Code.