fix(ci): force LF for diffview testdata fixtures on Windows

[johnjansen]

Problem

On Windows CI, TestDiffViewWidth/Split/WidthOf082 fails with a golden
mismatch: the closing } renders red (#d20f39, the catppuccin-latte
keyword color) instead of grey (#4c4f69, the text color). Example:
https://github.com/charmbracelet/crush/actions/runs/25949037144/job/76283113121

Root cause

The test embeds Go-source fixtures via go:embed:

//go:embed testdata/TestMultipleHunks.before
var TestMultipleHunksBefore string

These .before/.after files have no entry in .gitattributes, so on
Windows checkouts with the default core.autocrlf=true they get
converted to CRLF on disk. Chroma's Go lexer then sees }\r\n at EOF
and tokenizes the trailing brace inconsistently — at most widths the
renderer still picks the text color, but at width 82 the layout boundary
exposes a token that's been classified as a keyword.

Linux and macOS check out with LF, so the issue is invisible there. The
golden files are already protected via *.golden ... -text, but the
source fixtures that feed chroma are not.

Fix

Pin the embedded fixtures to LF in .gitattributes:

internal/ui/diffview/testdata/*.before text eol=lf
internal/ui/diffview/testdata/*.after  text eol=lf

This forces git to keep these files LF in the working tree on all
platforms, matching what's stored in the repo and what macOS / Linux
contributors already see.

Test plan

• go test ./internal/ui/diffview/... passes locally on macOS
  (baseline — was already passing).
• git check-attr eol internal/ui/diffview/testdata/*.before
  reports eol: lf after the change.
• CI on Windows passes — to be verified by this PR's checks.

No code change, no behavior change on Linux/macOS, no golden
regeneration needed.

• I have read CONTRIBUTING.md.

[johnjansen]

Closing as superseded / stale relative to v0.70.0.