docs,save: correct a stale TODO claim and rename encodeSnapshot (closes #27)

Four cleanups recorded as advisories during the PR #26 review and
deliberately kept out of it. No behaviour change.

The 2026-08-09 sig-leave (closes #12) TODO entry still argued in the
present tense that declining to save on SIGINT/SIGQUIT was also the safe
choice, "because AutoSave gob-encodes live state that the main goroutine
is still mutating, after removing the old file". Both halves stopped
being true with #24: the encode runs on the game goroutine and saveFile
is CreateTemp/Sync/Chmod/Rename with no Remove. The paragraph is now in
the past tense and marked superseded, pointing at the fix/autosave-race
entry, and says the split stands on C and on semantics alone — which is
what the current savesOnSignal comment says. The false claim was in the
#12 entry, not the #24 one; the latter's account of the old
remove-then-write is correctly historical and is untouched, as is the
err113 mention in the 2026-07-06 entry.

encodeSnapshot becomes writeSnapshotFile: it encodes, fsyncs, chmods
0400 and closes, and the old name claimed only the first of those. Its
doc comment now names all four and why the fsync is there.

TestAutoSaveOnSignalWhileInShellEscape used t.Error for a precondition,
so a save that was never taken fell through into assertRestorable, which
can then only report a second, derived failure. t.Fatal, matching the
identical assertion in the blocked-on-input test.

serviceAutoSaveRequest's doc comment carried a 24-column stub line
("The result is still a") left by an earlier edit. gofmt does not rewrap
comments, so fmt-check was legitimately green and nothing would have
caught it; the paragraph is rewrapped to the block's width.

Next Step deliberately not rotated: out-of-band issue work.

game/autosave_test.go

@@ -152,7 +152,7 @@ func TestAutoSaveOnSignalWhileInShellEscape(t *testing.T) {
 	<-st.entered
 
 	if !g.AutoSaveOnSignal(autoSaveWait) {
-		t.Error("the save was not taken while the game was in the shell escape")
+		t.Fatal("the save was not taken while the game was in the shell escape")
 	}
 
 	assertRestorable(t, g.FileName)