fix building C

docs/c-modules.md

@@ -244,6 +244,20 @@ audio_emscripten.c # Web/Emscripten
 
 ƿit selects the appropriate file based on the target platform.
 
+## Multi-File C Modules
+
+If your module wraps a C library, place the library's source files in a `src/` directory. Files in `src/` are compiled as support objects and linked into your module's dylib — they are not treated as standalone modules.
+
+```
+mypackage/
+  rtree.c       # module (exports js_mypackage_rtree_use)
+  src/
+    rtree.c      # support file (linked into rtree.dylib)
+    rtree.h      # header
+```
+
+The module file (`rtree.c`) includes the library header and uses `cell.h` as usual. The support files are plain C — they don't need any cell macros.
+
 ## Static Declarations
 
 Keep internal functions and variables `static`: