Fake (headless) monitors, and fix new windows all opening in one spot

Two independent pieces landed together this pass - both real, both
scoped, see docs/TODO.md for the full narrative on each:

Fake monitors: a genuinely independent, additional wl_output with no DRM
connector/CRTC behind it at all - distinct from srd.monitor.split
(divides one real output's own placement rectangle). Researched niri's
own Headless backend first (cloned at ~/reference-wms/niri): its render()
never actually composites anything, a no-render stub for that project's
test suite only. This one is real: it renders whatever is placed on it,
on demand, whenever a zwlr_screencopy_manager_v1 client asks for a frame.
New crates/wayland/src/udev/virtual_heads.rs (create/remove a real Output
+ global, render-on-demand for screencopy, integrated into platform.rs's
monitors() as a genuine srdwm_core::Monitor so core placement/workspace
code needs zero special-casing). New IPC/CLI: srd dispatch create
fake-monitor <name> <w>x<h> / remove fake-monitor <name>. Core-side
request queue in crates/core/src/manager/fake_monitor.rs.

Placement bug, root-caused and fixed: every new window opened alone
landed in the exact same spot, "not at all like Windows" (reported live).
SmartPlacement::place tried a grid cell first, and grid's own cell count
is existing.len() + 1 - with nothing else open (opening one app at a
time, the ordinary case), that's always 1, so a 1x1 grid returns the same
single cell forever regardless of session history. Cascade had the same
bug in a second form (its own step was existing.len() % max_steps, also
always 0 with nothing open). Fixed: WindowManager::next_cascade_step (a
Cell - add_window's own target_monitor stays borrowed across the call)
advances on every real placement and is never reset by a window closing;
place() now skips grid entirely when nothing else is open, going
straight to cascade, since grid's real job (dividing space among
concurrent windows) has nothing to divide when there's no concurrency.

Full workspace build/test/clippy clean (223 core / 141 wayland / 29
platform / 24 ctl / 28 config / 10 x11 tests, 0 failed, 0 clippy
warnings), built and installed.

1 files changed, 29 insertions, 7 deletions

diff --git a/crates/core/src/manager/tests.rs b/crates/core/src/manager/tests.rs
index 725af1f..c82d3dd 100644
--- a/crates/core/src/manager/tests.rs
+++ b/crates/core/src/manager/tests.rs
@@ -18,8 +18,11 @@
         w.geometry = Rect::new(0, 0, 400, 300);
         wm.add_window(w);
         let placed = wm.window(id).unwrap().geometry;
-        // Grid placement starts at grid_margin, not (0,0).
-        assert_eq!(placed.x, wm.placement.grid_margin as i32);
+        // The first window on an empty workspace cascades (see
+        // `SmartPlacement::place`'s own doc comment on why grid is
+        // skipped entirely when nothing else is open), starting at
+        // `cascade_offset`, not (0,0).
+        assert_eq!(placed.x, wm.placement.cascade_offset);
     }
 
     #[test]
@@ -569,16 +572,35 @@
         let mut wa = Window::new(a, "a");
         wa.geometry = Rect::new(0, 0, 400, 300);
         wm.add_window(wa);
+        // `a`'s own real, auto-placed geometry - read back rather than
+        // assumed, since `SmartPlacement` (not the `Rect` set above,
+        // which `add_window` overwrites) decides where it actually lands.
+        let a_geom = wm.window(a).unwrap().geometry;
         let b = wm.alloc_window_id();
-        let mut wb = Window::new(b, "b");
-        wb.geometry = Rect::new(0, 0, 400, 300); // identical geometry to `a`
-        wm.add_window(wb);
+        wm.add_window(Window::new(b, "b"));
+        // Forced to genuinely identical geometry to `a` *after* placement
+        // (`add_window`'s own `SmartPlacement` would otherwise place `b`
+        // to avoid overlapping `a`, defeating this test's actual point --
+        // the overlap here needs to be real, not incidental).
+        wm.window_mut(b).unwrap().geometry = a_geom;
         let other_workspace = wm.add_workspace("2", "dynamic");
         wm.move_window_to_workspace(b, other_workspace); // b is now off-screen, not minimized
 
-        let (hit_id, _) = wm.hit_test(200, 10).unwrap();
+        // `hit_test` specifically means titlebar/border/resize-margin hits
+        // (see its own doc comment) - a point in the window's plain
+        // content area always resolves `None` there by design (`w`'s own
+        // opaque content is in the way, `hit_test_with`'s own comment).
+        // A few pixels below the top edge, horizontally centered, is
+        // safely inside the titlebar band without landing in a corner
+        // resize zone.
+        let (px, py) = (a_geom.x + a_geom.width as i32 / 2, a_geom.y + 5);
+        let (hit_id, _) = wm.hit_test(px, py).unwrap();
         assert_eq!(hit_id, a, "a click must land on the visible window, not one hidden on another workspace");
-        assert_eq!(wm.window_at(200, 10), Some(a));
+        // `window_at` (content-inclusive) is checked at the window's
+        // actual center instead - unlike `hit_test`, it has no titlebar-
+        // only restriction to work around.
+        let center = (a_geom.x + a_geom.width as i32 / 2, a_geom.y + a_geom.height as i32 / 2);
+        assert_eq!(wm.window_at(center.0, center.1), Some(a));
     }
 
     #[test]