chore: add .gitattributes and normalize line endings to LF

Eleven Python files were committed with CRLF, socketdev/__init__.py among them.
Any tooling that reads and rewrites one of those files converts it to LF on the
way out, so a two-line edit arrives as a whole-file diff with the real change
buried in it. That happened while writing the enum change in this same branch.

This normalizes all of them once and pins the setting so it cannot recur.
Reviewable with `git diff -w`, which shows only .gitattributes: no file content
changed, and the unit suite is unaffected.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: lelia

.gitattributes

@@ -0,0 +1,29 @@
+# Normalize line endings to LF in the repository.
+#
+# Eleven Python files were committed with CRLF, socketdev/__init__.py among
+# them. Any tooling that reads and rewrites one of those files converts it to
+# LF on the way out, which turns a two-line change into a whole-file diff and
+# buries the actual edit. Normalizing once and pinning the setting here stops
+# that from recurring.
+* text=auto
+
+*.py text eol=lf
+*.pyi text eol=lf
+*.md text eol=lf
+*.rst text eol=lf
+*.toml text eol=lf
+*.cfg text eol=lf
+*.yml text eol=lf
+*.yaml text eol=lf
+*.json text eol=lf
+*.sh text eol=lf
+*.lock text eol=lf
+
+# Byte-exact, never line-ending-normalized.
+*.png binary
+*.jpg binary
+*.jpeg binary
+*.gif binary
+*.ico binary
+*.whl binary
+*.gz binary