fix(gtk): install prebuilt modules without development files

README.md

@@ -77,8 +77,8 @@ make check-stress   # randomized, capacity, collision, failure, and restart test
 tests. `make docker-check` force-rebuilds and runs all three tiers in the
 container; run `make docker-image` first after changing `Dockerfile`.
 
-Native Linux builds require a C toolchain, Make, pkg-config, Python 3, and
-cmp/diff, plus:
+Native Linux source builds require a C toolchain, Make, pkg-config, Python 3,
+and cmp/diff, plus development files for:
 
 - Plan 9 port (`9c`, `9l`, `libthread`, and `libbio`);
 - D-Bus development files;
@@ -133,10 +133,17 @@ export GTK_IM_MODULE=strans
 GLFW_IM_MODULE=ibus kitty
 ```
 
-The GTK module directory is derived from GTK's pkg-config `libdir` and binary
-version. It can be overridden with `GTK_MODULE_DIR`; `DESTDIR` staging is
-supported and deliberately skips the host module-cache update. Remove a
-native installation with:
+Building `gtk/im-strans.so` requires GTK development files. Installing an
+already-built, up-to-date module from this build tree does not: the install
+target first uses an explicit `GTK_MODULE_DIR`, then GTK's pkg-config
+variables, and finally the module directory reported by the GTK runtime's
+`gtk-query-immodules-3.0`. Packagers and cross builds should set
+`GTK_MODULE_DIR` explicitly. A prebuilt module must still match the target
+architecture and library ABI.
+
+`DESTDIR` staging is supported and deliberately skips the host module-cache
+update. A native install needs the GTK runtime query utility so it can refresh
+that cache. Remove a native installation with:
 
 ```sh
 doas make -C gtk uninstall