fix: take the signal-time autosave on the game goroutine (closes #24)

The SIGHUP/SIGTERM handler gob-encoded the live game tree from the signal
goroutine while the game goroutine was mid-turn mutating it, and AutoSave
removed the save file before encoding — so the failure mode was not a
stale save but a deleted one followed by a possibly torn replacement,
with a window in which the player had neither. The suite has run under
-race since 2026-08-09 and was green because nothing had ever driven the
turn loop concurrently with a signal: evidence of untested, not of safe.

The handler no longer writes anything. AutoSaveOnSignal posts a request,
wakes the input read, and waits up to signalSaveTimeout for the game
goroutine to take it; the encode runs on the goroutine that owns the
state, at the three points where that goroutine can sit: between turns
(command), on waking from a blocked readchar, and while parked in the `!`
shell escape (runShellEscape, which now runs the shell on a helper
goroutine so a hangup during it still rescues the game).

Blocked on input is the case that matters — a dropped connection lands
while the player is thinking, so a flag checked only between turns would
never be looked at. Terminal.ReadChar therefore returns (byte, bool),
with ok false meaning "woken by Interrupt, no key", and term.Tcell posts
a tcell.EventInterrupt onto tcell's own event queue to unpark PollEvent.
readchar services the request and reads again, so no caller sees it.

saveFile writes a temporary file in the save's own directory, fsyncs it
and renames it over the target instead of truncating in place, so a save
that fails — or never happens because the deadline ran out — leaves the
player's previous save whole.

The SIGINT/SIGQUIT no-save decision and the single-signal-read ordering
guarantee are untouched. pendingSaver reads the game out from under its
mutex rather than delegating with it held, because the delegated call now
blocks until the save is taken.

game/screen.go

@@ -14,8 +14,15 @@ type Terminal interface {
 	// Render blits the window to the device.
 	Render(w *Window)
 	// ReadChar blocks for the next key, translated to Rogue's input bytes
-	// (arrows become hjkl, control keys their C0 codes).
-	ReadChar() byte
+	// (arrows become hjkl, control keys their C0 codes). ok is false when
+	// the read was woken by Interrupt instead of by a key, which is how a
+	// signal-triggered autosave reaches a game parked on input; the byte
+	// is meaningless then.
+	ReadChar() (ch byte, ok bool)
+	// Interrupt wakes a ReadChar that is blocked waiting for a key. It is
+	// the one Terminal method called from another goroutine, so an
+	// implementation must be safe to call concurrently with ReadChar.
+	Interrupt()
 	// Fini restores the device to its pre-game state (curses endwin). The
 	// game calls it on its way out, since one game run is one process.
 	Fini()
@@ -223,6 +230,15 @@ func (s *Screen) Fini() {
 	}
 }
 
+// Interrupt wakes a device read that is blocked waiting for a key, if
+// there is a device. Called from the signal goroutine; everything else on
+// Screen belongs to the game goroutine.
+func (s *Screen) Interrupt() {
+	if s.term != nil {
+		s.term.Interrupt()
+	}
+}
+
 // RefreshWin pushes an arbitrary window to the device (curses wrefresh).
 func (s *Screen) RefreshWin(w *Window) {
 	if s.term != nil {