Add .gitignore for build artifacts

Ignore compiled objects (.a, .o, .sym), executables, and build directories
to keep the repository clean.

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

.gitignore

@@ -0,0 +1,19 @@
+# Compiled object files
+*.o
+
+# Archive/library files
+*.a
+
+# Symbol files
+*.sym
+
+# Executables
+plvm
+xplasm
+run6502
+
+# Build output directories
+src/rel/
+
+# macOS
+.DS_Store