feat(simulation): add YAML to RON converter tool (#403)

Build-time converter reads content YAML and emits RON format. Lives in
tooling/content-converter/ as standalone Rust crate. Runs via make
content-ron. Supports --dry-run, --verbose, --content-type filtering.
Not on critical path — engine consumes YAML in v0.1, RON is future-
proofing for runtime performance.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.gitignore

@@ -1,6 +1,9 @@
 # Build and cache
 .cache/
 server/target/
+tooling/content-converter/target/
+tooling/line-previewer/target/
+content-ron/
 
 # Godot client (further ignores managed by client team)
 client/.godot/