wm: F1 spawns a windowed shell — real shell, not a duplicate

Architecture:
- vt: ACTIVE terminal redirect — when set, all gop_print!/put_char/write_str
  go there instead of HOME.  Terminal::from_buffer() renders into a
  WM-owned pixel buffer.
- tty: read_line_from_queue() — canonical-mode line input from the task's
  own message queue (TaskMsg::Key).  Returns None on Close.
- shell: run_windowed() — same shell as the console but reads from the
  message queue.  Returns on ^D or window close.
- wm: F1 calls SPAWN_TERMINAL_CB (set by rsdos).  No shell logic in the WM.
- rsdos: terminal_task callback + terminal_entry task — creates a 640x400
  chrome window, wires up Terminal::from_buffer, sets ACTIVE, runs
  shell::run_windowed(), cleans up on exit.
- shell: 'term' debug command that invokes the callback directly and
  logs each step to serial.

1 files changed, 15 insertions, 1 deletions

diff --git a/rsnix/src/wm.rs b/rsnix/src/wm.rs
index 2641b69..05781e8 100644
--- a/rsnix/src/wm.rs
+++ b/rsnix/src/wm.rs
@@ -394,8 +394,22 @@ pub fn handle_key(ev: KeyEvent) -> bool {
     }
 }
 
+static TERM_COUNT: core::sync::atomic::AtomicUsize = core::sync::atomic::AtomicUsize::new(0);
+
+/// Callback set by rsdos to spawn a terminal task.  Takes (title, w, h).
+pub static mut SPAWN_TERMINAL_CB: Option<fn(&str, usize, usize)> = None;
+
 fn spawn_terminal() {
-    crate::serial_println!("wm: F1 (spawn terminal) not implemented yet");
+    let Some(cb) = (unsafe { SPAWN_TERMINAL_CB }) else {
+        crate::serial_println!("wm: SPAWN_TERMINAL_CB not set");
+        return;
+    };
+    let n = TERM_COUNT.fetch_add(1, Ordering::Relaxed) + 1;
+    let title = alloc::format!("term {}", n);
+    let (cw, ch) = gop::pixel_dims();
+    let w = 640.min(cw);
+    let h = 400.min(ch);
+    cb(&title, w, h);
 }
 
 fn remove_from_zorder(id: usize) {