kernel: fix asset hot-reload regression (watch the whole asset dir, not the .rml basename)

The watch_file refactor (35e5d32) moved the substrate's UI-asset hot-reload onto
the shared FileWatcher but armed a BASENAME watch on the document's .rml file only.
The dock's styling lives in a separately-<link>ed dock.rcss, so editing it (the
common case) never matched the watch — asset hot-reload silently stopped working on
the real seat (no "dev hot-reload ON" line, no reload on save), while config
watching kept working. The ui_reload_surface() seam test passed because it bypassed
the real inotify->reload path.

Fix: FileWatcher::add_dir watches the document's whole DIRECTORY (so any .rml/.rcss
in it triggers the surface reload); the substrate uses it and restores the
"dev hot-reload ON (inotify watching asset dir '...')" log. Added an END-TO-END
test mirroring the dock (a doc that <link>s a separate .rcss, real inotify event,
wl_event_loop pumped, assert the document actually reloaded) — fails on the buggy
code, passes now; no more relying on the seam.

Real-seat verified: editing dock.rcss now reloads the live dock (border-radius +
background-color changes apply on save). kernel 59 cases/260 assertions green on
build + build-asan, no new suppressions. Edits confined to packages/kernel/.

1 files changed, 16 insertions, 1 deletions

diff --git a/packages/kernel/src/file_watcher.cpp b/packages/kernel/src/file_watcher.cpp
index ae292c2..c747fe4 100644
--- a/packages/kernel/src/file_watcher.cpp
+++ b/packages/kernel/src/file_watcher.cpp
@@ -109,6 +109,19 @@ auto FileWatcher::add(const std::string& path, std::function<void()> on_change,
     return FileWatch(this, token);
 }
 
+auto FileWatcher::add_dir(const std::string& dir, std::function<void()> on_change, ExtensionId who)
+    -> FileWatch {
+    if (!ensure_started()) {
+        return FileWatch{};
+    }
+    std::string d = dir.empty() ? std::string(".") : dir;
+    const Token token = ++next_token_;
+    // Empty basename => match ANY file change in `d`.
+    entries_.emplace(token, Entry{std::move(d), std::string{}, std::move(on_change), who});
+    arm_dir(entries_.at(token).dir);
+    return FileWatch(this, token);
+}
+
 void FileWatcher::remove_watch(Token token) noexcept {
     auto it = entries_.find(token);
     if (it == entries_.end()) {
@@ -164,7 +177,9 @@ void FileWatcher::on_readable() {
     std::vector<Token> to_fire;
     for (const auto& [token, e] : entries_) {
         for (const auto& [dir, name] : changed) {
-            if (e.dir == dir && e.basename == name) {
+            // A file-specific entry matches its basename; a directory entry
+            // (empty basename) matches ANY file change in its dir.
+            if (e.dir == dir && (e.basename.empty() || e.basename == name)) {
                 to_fire.push_back(token);
                 break;
             }