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.

Running the shell on a helper goroutine would also have moved
term.Tcell.ShellEscape's panic on a failed Screen.Resume onto it, and a
panic at the top of any goroutine terminates the process without running
the deferred calls of the others — including cmd/rogue/main.go's
`defer t.Fini()`. The tty would have been left raw on precisely the path
where the terminal is already broken, which is issue #12's failure on a
path this change created. runShellEscape therefore recovers the helper's
panic and re-raises it on the game goroutine, whose stack has the restore
in it, so "every path restores the terminal via Terminal.Fini before
exiting" stays true.

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.

What the handoff guarantees is stated exactly rather than flatteringly:
the encode runs on the state-owning goroutine, so the snapshot is
internally consistent and restorable, but it is not necessarily taken
between commands. readchar is reached from mid-command prompts (--More--,
askOverwrite, getStr, the direction and pack prompts) and the command has
already mutated state by then, so a save taken there freezes that command
half applied and restoring loses the rest of it.

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.

MEMORY.md

@@ -10,7 +10,36 @@ unlikely error returns through game code — e.g. write-side Close/encode failur
 where continuing would mean corrupt state. Return errors where a caller
 genuinely handles them (save-file prompts, restore validation). Reserve
 deliberate `_ =` discards for true best-effort paths (scorefile writes,
-signal-time autosave), always with a comment saying why.
+`Terminal.Interrupt`'s post to a full event queue), always with a comment saying
+why.
+
+Signal-time autosave used to be on that list and no longer is (issue #24). It is
+best effort in the sense that nothing can be reported to a player whose terminal
+is already going away, but the outcome is a value, not a discard: the signal
+goroutine calls `AutoSaveOnSignal`, which hands the save to the game goroutine —
+the only one allowed to touch game state — and returns whether it was taken
+before the deadline. The game answers between turns (`command`), while parked
+waiting for a key (`readchar`), and while parked in the `!` shell escape
+(`runShellEscape`). `saveFile` writes a temporary file and renames it over the
+target, so a save that fails or never happens leaves the player's previous save
+whole; never reintroduce a `Remove` before the write in `autoSave`, and never
+encode game state from any goroutine but the game's.
+
+Do not upgrade that into "the snapshot is always taken between commands" — it is
+not. What is true is that the encode runs on the state-owning goroutine, so the
+snapshot is internally consistent and restorable. `readchar` is reached from
+mid-command prompts (`--More--`, `askOverwrite`, `getStr`, direction and pack
+prompts) and the command has already mutated state by then, so a save taken
+there freezes that command half applied and the player loses the rest of it on
+restore. That is acceptable and documented; the false stronger claim was caught
+in review of PR #26 and must not come back.
+
+Related, and easy to reintroduce: work moved onto a helper goroutine must not be
+allowed to panic there. A panic at the top of any goroutine kills the process
+without running the other goroutines' defers, including `cmd/rogue/main.go`'s
+`defer t.Fini()`, which is what leaves a raw tty (issue #12). `runShellEscape`
+recovers its helper's panic and re-raises it on the game goroutine for exactly
+that reason.
 
 C's exit() calls are not unwound: one game run is one process, so myExit
 (game/rip.go) restores the terminal via Terminal.Fini and calls os.Exit(0), and