Keep shell scripts LF so a Windows clone can run the Linux path

`* text=auto` and nothing else, so a Windows clone writes every shell script with
carriage returns. The documented Linux path is a container that runs one of them,
and it dies in under a second with a complaint about an interpreter named
`bash\r`. Nothing in that failure names line endings, and the same clone builds
and tests fine everywhere else, so it reads as a broken image.

`scripts/build-quic-native.sh` is the only shell script in the repository and the
only file on those paths that an interpreter reads by shebang; the mutation
runner is Node and handles carriage returns itself, and the Dockerfile builds
either way.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

.gitattributes

@@ -1,2 +1,10 @@
 # Auto detect text files and perform LF normalization
 * text=auto
+
+# Shell scripts keep LF in the working tree as well as in the index. Left to
+# `text=auto`, a Windows clone writes them with carriage returns, and the
+# documented Linux path then dies inside the container in under a second: the
+# shebang reads as the interpreter `bash\r`, which does not exist. Nothing about
+# that failure names line endings, and the same clone builds and tests fine
+# everywhere else, so it reads as a broken image rather than a broken checkout.
+*.sh text eol=lf