Initial commit — Stage 0 project foundation

- Cargo workspace with engine, editor, tests crates
- oxide-editor binary stub (builds and runs cleanly)
- install.sh for Linux system installation
- README.md with logo, build and install instructions
- GPLv3 LICENSE
- .gitignore

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

tests/src/lib.rs

@@ -0,0 +1,11 @@
+//! Oxide Engine — integration test harness.
+//!
+//! End-to-end tests are added here as each stage is completed.
+
+#[cfg(test)]
+mod stage0 {
+    #[test]
+    fn workspace_builds() {
+        // Trivial smoke test: if this file compiles, Stage 0 foundation is in place.
+    }
+}