Keep a file's own line endings when saving it

[ewraj]

Foundation fix F10 from PHASE_6_OVERVIEW.md (#17). This is a bug on main today, independent of Phase 6.

The bug

CodeMirror splits a document on \r\n, \r and \n, and doc.toString() always joins it back with \n. CodeView hands doc.toString() to the session, and save writes it through the adapter. As a result, editing one character of a Windows (CRLF) file in a picked folder rewrites every line ending on disk, and the whole file shows as changed in version control.

The fix

The editor keeps normalising to \n, which is what keeps positions and anchors simple. The file's line break is restored at the one place text leaves for disk:

• New src/sources/lineBreaks.ts: dominantLineBreak, withLineBreak and keepLineBreaks.
• CachingAdapter.writeFile and StoredAdapter.writeFile re-apply the dominant break of the cached text the edit replaces. Both the disk and the cache get the same bytes, so the next save sees CRLF again.
• Notebooks are excluded. What's edited there is the generated \n reading view, and the file on disk is rebuilt as JSON by sourceToNotebook.
• CachingAdapter is now exported so the save path can be tested directly.

Known limit: a file that mixes endings is written with the one it uses most. That's much closer to the original than today's rewrite to all \n. Keeping each line's own ending through arbitrary edits would mean mapping them through every change, which isn't worth it for what is almost always an accident.

Tested

• Table-driven tests for detection (ties go to \n, and \r\n counts as one break) and for conversion.
• Open CRLF → edit in a real CodeMirror EditorState → save through CachingAdapter: the bytes written to "disk" and to the cache are still CRLF. A second save keeps them. A Unix file stays Unix.
• With the fix temporarily removed, the two CRLF tests fail, so the test does catch the bug.
• The CRLF inputs are built as strings, not fixture files, because of autocrlf.
• npm run typecheck, npm test (283 tests), npm run build and npm run test:e2e (29 tests, Chromium) all pass locally.

Not covered: an e2e test with a real picked folder. Playwright can't drive showDirectoryPicker, and the existing suite seeds IndexedDB instead.

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