feat: add make lint target for diff patch validation

Makefile

@@ -1,4 +1,4 @@
-.PHONY: build clean validate test all schemas schema-validate
+.PHONY: build clean validate test all schemas schema-validate lint
 
 DIST := dist
 SRC := src
@@ -21,6 +21,12 @@ endif
 	uv run x4cat extract "$(X4_GAME_DIR)" -o $(SCHEMAS) -g '*.xsd'
 	@echo "Schemas extracted to $(SCHEMAS)/"
 
+lint:
+ifndef X4_GAME_DIR
+	$(error X4_GAME_DIR not set — e.g. make lint X4_GAME_DIR="/path/to/X4 Foundations")
+endif
+	uv run x4cat validate-diff "$(X4_GAME_DIR)" $(SRC)
+
 schema-validate:
 	uv run pytest tests/test_mod.py::TestSchema -q
 

README.md

@@ -35,6 +35,18 @@ make test             # full test suite (includes build verification)
 make clean            # remove dist/
 ```
 
+## Diff patch linting
+
+Validate that all XML diff patches in `src/` have XPath selectors that match
+the base game files they target:
+
+```bash
+make lint X4_GAME_DIR="/path/to/X4 Foundations"
+```
+
+Reports mismatches with file path, line number, and the failing XPath.
+Skipped gracefully when no diff patches exist in `src/`.
+
 ## Schema validation
 
 Optionally extract XSD schemas from your X4 install for full schema validation