fix building

docs/c-modules.md

@@ -255,6 +255,8 @@ If your package is at `/path/to/mypkg`, this becomes `-I/path/to/mypkg/sdk/publi
 
 Absolute paths are passed through unchanged.
 
+The build system also auto-discovers `include/` directories — if your package has an `include/` directory, it is automatically added to the include path. No need to add `-I$PACKAGE/include` in cell.toml.
+
 ### Library paths
 
 Relative `-L` paths work the same way:
@@ -285,9 +287,13 @@ Available targets: `macos_arm64`, `macos_x86_64`, `linux`, `linux_arm64`, `windo
 
 ### Sigils
 
-Use `$LOCAL` in flags to refer to the `.cell/local` directory (for prebuilt libraries):
+Use sigils in flags to refer to standard directories:
+
+- `$LOCAL` — absolute path to `.cell/local` (for prebuilt libraries)
+- `$PACKAGE` — absolute path to the package root
 
 ```toml
+CFLAGS = "-I$PACKAGE/vendor/include"
 LDFLAGS = "-L$LOCAL -lmyprebuilt"
 ```
 
@@ -495,3 +501,32 @@ static int module_state = 0;
 ```
 
 This prevents symbol conflicts between packages.
+
+## Troubleshooting
+
+### Missing header / SDK not installed
+
+If a package wraps a third-party SDK that isn't installed on your system, the build will show:
+
+```
+module.c: fatal error: 'sdk/header.h' file not found (SDK not installed?)
+```
+
+Install the required SDK or skip that package. These warnings are harmless — other packages continue building normally.
+
+### CFLAGS not applied
+
+If your `cell.toml` has a `[compilation]` section but flags aren't being picked up, check:
+
+1. The TOML syntax is valid (strings must be quoted)
+2. The section header is exactly `[compilation]` (not `[compile]` etc.)
+3. Target-specific sections use valid target names: `macos_arm64`, `macos_x86_64`, `linux`, `linux_arm64`, `windows`
+
+### API changes from older versions
+
+If C modules fail with errors about function signatures:
+
+- `JS_IsArray` takes one argument (the value), not two — remove the context argument
+- Use `JS_GetPropertyNumber` / `JS_SetPropertyNumber` instead of `JS_GetPropertyUint32` / `JS_SetPropertyUint32`
+- Use `JS_NewString` instead of `JS_NewAtomString`
+- There is no `undefined` — use `JS_IsNull` and `JS_NULL` only