Add desktop services: open URI, notify, appearance, bell, and paths

wsap_term keeps per-platform code for opening links, notifications, the
bell, high-contrast detection, the executable and bundle paths, and the
default locale. These now live behind <yasl/desktop.h>.

On Linux, src/platform/freedesktop serves both backends. Opening a URI
runs xdg-open from PATH through a double fork, so the opener gets default
signal dispositions, an empty mask, a new session, and none of the
application's descriptors. Notifications and the Settings portal use the
session bus through libdbus-1, an optional dependency behind -Ddbus.
Appearance falls back to GTK_THEME without a portal. The bell uses
xdg-system-bell on Wayland and the XKB bell on X11.

Cocoa implements the same calls with NSWorkspace, UserNotifications, the
effective appearance, NSBeep, the bundle, and CFLocale. That code is not
built or run here; it needs a macOS check.

The desktop test runs under dbus-run-session against a fake notification
server and portal, and uses a fake xdg-open to check argument passing,
signal state, and descriptor inheritance.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

1 files changed, 39 insertions, 2 deletions

diff --git a/docs/guide.md b/docs/guide.md
index 61583b1..7f02ab7 100644
--- a/docs/guide.md
+++ b/docs/guide.md
@@ -1,8 +1,8 @@
 # Application guide
 
 The contracts in [yasl.h](../include/yasl/yasl.h), [loop.h](../include/yasl/loop.h),
-[timing.h](../include/yasl/timing.h), and [native.h](../include/yasl/native.h) are
-the reference for individual calls. All examples here use the display's opening
+[timing.h](../include/yasl/timing.h), [desktop.h](../include/yasl/desktop.h), and
+[native.h](../include/yasl/native.h) are the reference for individual calls. All examples here use the display's opening
 thread, which must be the main thread on macOS. Userdata stays owned by the
 application. Copy borrowed data before retaining it beyond its documented life.
 
@@ -363,6 +363,43 @@ and restores the previous action on unwatch. YASL leaves thread signal masks
 alone: a watched signal must stay unblocked in at least one thread. Only one
 loop can watch a given signal at a time.
 
+## Desktop services
+
+Include `<yasl/desktop.h>` for services that sit next to windowing. Display and
+surface calls use the display thread; path and locale calls work from any
+thread.
+
+| Call | Linux | Cocoa |
+| --- | --- | --- |
+| `yasl_display_open_uri()` | `xdg-open` from `PATH` | NSWorkspace |
+| `yasl_display_notify()` | Notifications service over D-Bus | User notifications; app bundle required |
+| `yasl_display_appearance()` | Settings portal over D-Bus, then `GTK_THEME` | Effective appearance and increase-contrast setting |
+| `yasl_surface_bell()` | Wayland xdg-system-bell; X11 XKB bell | `NSBeep` |
+| `yasl_executable_path()` | `/proc/self/exe` | Executable path, resolved |
+| `yasl_resource_dir()` | Always zero | App bundle resources |
+| `yasl_default_locale()` | `LC_ALL`, `LC_CTYPE`, `LANG`, then `C.UTF-8` | Current language and region |
+
+`yasl_display_open_uri()` checks only that the URI has a scheme. Decide which
+schemes and hosts your application trusts before calling it, for example
+before opening a link from untrusted text. On Linux the opener starts in its
+own session with default signal handling and none of the application's
+descriptors above stderr. D-Bus services need YASL built with `dbus-1`; without
+it, notifications return false and appearance falls back to `GTK_THEME`.
+Appearance changes send no notification, so query again when a window gains
+focus.
+
+Functions that fill a buffer return the full length, excluding the NUL. A
+result less than the buffer size means the string was written. Otherwise the
+buffer is unchanged, so call again with a larger one:
+
+```c
+char path[4096];
+size_t len = yasl_executable_path(path, sizeof(path));
+if (len == 0 || len >= sizeof(path)) {
+    /* No path, or a longer buffer is needed. */
+}
+```
+
 ## Native interoperability
 
 Portable CPU drawing needs no native headers. Opt into `<yasl/native.h>` and