git: union-merge Cargo.toml to end the example-list conflict cascade

Every topic branch that adds a [[example]] entry collides on the same region
of Cargo.toml, so each merge dirties the next chained PR. The built-in `union`
merge driver keeps both sides of an additive conflict hunk — exactly right for
this append-only list — and GitHub's merge honors it. Once this lands on master,
subsequent example-adding PRs merge with no manual conflict resolution.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MLBnPuScZy6w9di2QEjsXM

Author: claude

.gitattributes

@@ -0,0 +1,5 @@
+# Additive edits to the [[example]] list collide constantly across topic
+# branches. The built-in union merge driver keeps both sides of a conflict
+# hunk, auto-resolving append-only lists. (Distinct dependency-line edits are
+# caught by CI, not silently merged wrong.)
+Cargo.toml merge=union