chore: ignore .DS_Store and generated lecture output

slides/dist/ (the single-file bundle) and docs/lab/handout/class.pdf (one
sheet per student) are build products.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

.gitignore

@@ -1,3 +1,5 @@
 /target
 __pycache__/
 slides/reveal.js/
+slides/dist/
+.DS_Store

docs/lab/handout/.gitignore

@@ -5,3 +5,4 @@
 *.fdb_latexmk
 *.synctex.gz
 build.log
+class.pdf