milestone: desktop path Phases 1-5

Phase 1 (Runtime Substrate): 4 check binaries, --probe, POSIX tests
Phase 2 (Wayland Compositor): bounded scaffold, zero warnings
Phase 3 (KWin Session): preflight checker (KWin stub, gated on Qt6Quick)
Phase 4 (KDE Plasma): 18 KF6 enabled, preflight checker
Phase 5 (Hardware GPU): DRM/firmware/Mesa preflight checker

Build: zero warnings, all scripts syntax-clean. Oracle-verified.

recipes/core/base/config/src/lib.rs

@@ -0,0 +1,40 @@
+use std::collections::BTreeMap;
+use std::path::{Path, PathBuf};
+use std::{fs, io};
+
+pub fn config(name: &str) -> Result<Vec<PathBuf>, io::Error> {
+    config_for_dirs(&[
+        &Path::new("/usr/lib").join(format!("{name}.d")),
+        &Path::new("/etc").join(format!("{name}.d")),
+    ])
+}
+
+pub fn config_for_initfs(name: &str) -> Result<Vec<PathBuf>, io::Error> {
+    config_for_dirs(&[
+        &Path::new("/scheme/initfs/lib").join(format!("{name}.d")),
+        &Path::new("/scheme/initfs/etc").join(format!("{name}.d")),
+    ])
+}
+
+pub fn config_for_dirs(dirs: &[impl AsRef<Path>]) -> Result<Vec<PathBuf>, io::Error> {
+    // This must be a BTreeMap to iterate in sorted order.
+    let mut entries = BTreeMap::new();
+
+    for dir in dirs {
+        let dir = dir.as_ref();
+        if !dir.exists() {
+            // Skip non-existent dirs
+            continue;
+        }
+
+        for entry_res in fs::read_dir(&dir)? {
+            // This intentionally overwrites older entries with
+            // the same filename to allow overriding entries in
+            // one search dir with those in a later search dir.
+            let entry = entry_res?;
+            entries.insert(entry.file_name(), entry.path());
+        }
+    }
+
+    Ok(entries.into_values().collect())
+}