update docs for compile chain

docs/testing.md

@@ -192,6 +192,36 @@ Failures saved to tests/fuzz_failures/
 
 Saved failure files are valid `.cm` modules that can be run directly or added to the test suite.
 
+## Compile-Time Diagnostics Tests
+
+The `tests/compile.cm` test suite verifies that the type checker catches provably wrong operations at compile time. It works by compiling source snippets through the pipeline with `_warn` enabled and checking that the expected diagnostics are emitted.
+
+```javascript
+var shop = use('internal/shop')
+var streamline = use('streamline')
+
+function get_diagnostics(src) {
+  fd.slurpwrite(tmpfile, stone(blob(src)))
+  var compiled = shop.mcode_file(tmpfile)
+  compiled._warn = true
+  var optimized = streamline(compiled)
+  if (optimized._diagnostics == null) return []
+  return optimized._diagnostics
+}
+```
+
+The suite covers:
+- **Store errors**: storing named property on array, numeric index on record, property/index on text, push on text/record
+- **Invoke errors**: invoking null, number, text
+- **Warnings**: named property access on array/text, record key on record
+- **Clean code**: valid operations produce no diagnostics
+
+Run the compile diagnostics tests with:
+
+```bash
+pit test compile
+```
+
 ## Test File Organization
 
 Tests live in the `tests/` directory of a package: