Add a Mac as the third system, beside Wayland and X11

Dikte already chose its clipboard programs once instead of in every
function; macOS joins that table rather than adding a branch to each one.
A Mac copies through pbcopy and presses Cmd+V straight into CoreGraphics,
records through AVFoundation, and asks Carbon for its global shortcuts.

The three tables are paste.Desktop, audio.Sound, and the pair of
predicates in hotkey.py. Each reads sys.platform inside the chooser, so a
test can stand somewhere else: 697 of the 737 tests now run on any
machine, the Wayland and X11 halves included, and the suite passes whole
whichever system it is run on.

Two things a Mac does not have needed saying rather than pretending:
there is no shortcut registry to install into, so Settings offers no
Install button and the listener is the mechanism instead of a fallback;
and nothing is offered as the sound the speakers are playing, so a
meeting needs BlackHole or Loopback and says so. The KDE-only labels
around them were already wrong on GNOME, and now name whichever desktop
is there.

Co-authored-by: firat <[email protected]>

CONTRIBUTING.md

@@ -31,6 +31,7 @@ directories, resets the language, and puts them back afterwards.
 | A reply that fails | `http_error(429)`, `url_error()`, `raw_body("not json")` |
 | Reading what was sent | `sent_json(request)`, `multipart_fields(request)` |
 | A program on the PATH | `only_these_tools("pactl", "wl-copy")` |
+| Standing on another system | `mock.patch.object(sys, "platform", "darwin")` |
 | Audio | `silence()`, `tone()`, `speech()`, `stereo()`, `make_wav()` |
 | A settings object | `self.config(cleanup_enabled=False)` |
 
@@ -52,30 +53,35 @@ forgets fails rather than hangs.
 
 ## Another platform
 
-Most of what Dikte does is not desktop-specific, and the tests are split along
-that line. 511 of them pass anywhere: transcription, cleanup, the config file,
-the history, the agent, the command line, the timeline of a meeting. The
-remaining 59 cover what Dikte *is* on this desktop, and carry `@linux_only`
-from `tests.support`: PipeWire capture and the pactl device list, wl-clipboard
-and ydotool, KDE's shortcut file and the `/dev/input` listener.
+Three systems are supported: Wayland, X11 and macOS. Each one is a named entry
+in a table, and one chooser picks between them, so a fourth adds an entry and a
+line rather than a branch inside every function. The three tables are
+`paste.Desktop` (clipboard and key press), `audio.Sound` (capture and the device
+lists) and the `_macos()`/`_gnome()` pair in `hotkey.py`. Keep `sys.platform`
+inside the chooser and read it there every time: a constant settled at import is
+one no test can stand somewhere else.
 
-Mark a test `@linux_only` when it would fail on a machine that never had those
-programs. Do not mark one because it happens to be convenient: a test that
-quietly stops running on the platform you are porting to protects nothing.
-
-The other half of a port is where the branch goes. Keep `sys.platform` out of
-the middle of a function; make the public name a chooser and give each platform
-its own function underneath:
+The tests are split along the same line, and almost none of them are skipped.
+697 of the 737 run on any machine, including every line of the Wayland, X11 and
+macOS backends: the programs are faked at `shutil.which`, the frameworks at the
+one function that loads them. A test class says which system it is standing on
+rather than avoiding the question:
 
 ```python
-def copy(text):
-    return _copy_macos(text) if sys.platform == "darwin" else _copy_wayland(text)
+class MacOS(ClipboardContract, DikteTest):
+    platform = "darwin"
+    here = paste.MACOS
 ```
 
-Then each platform's test calls its own function directly and passes everywhere,
-and adding a third one leaves the first two's tests alone. An `if` buried inside
-`copy()` forces every existing test to patch `sys.platform` instead, and the
-next port breaks all of them.
+so the Linux half is checked on a Mac and the macOS half on Linux, and a change
+to a chooser cannot quietly break the platform nobody is sitting at. What the
+systems owe in common is written once as a contract class and subclassed by each
+of them.
+
+The 40 that do carry `@linux_only` are the ones that would need the real thing:
+the `/dev/input` listener, KDE's shortcut file, GNOME's gsettings. Mark a test
+that way only when faking it would leave nothing to test. A test that quietly
+stops running on the platform you are porting to protects nothing.
 
 ## What a pull request should carry