test: drive the autosave race test to a condition, not a turn count (closes #36)

TestAutoSaveOnSignalRacesTurnLoop failed intermittently under load. The
captured failure text settles what it was: driveUntilDone's

    t.Fatal("the turn loop ran out of turns before the saves were taken")

with no WARNING: DATA RACE anywhere in the log. The handoff is fine; the
test's own drive loop ran out of its fixed 1000-turn budget first.

Confirmed by instrumenting the loop to report the turns it actually
used. The count tracks scheduling pressure and nothing else: about
60-120 turns at host load ~57 with the whole machine to spread over, 418
at GOMAXPROCS=4, 539 and 655 at 2 and 1, and past 1000 under the doubled
load of the verbose rerun the test target performs after a failure. The
turns spent between one save being answered and the next request
arriving are not work, they are the saving goroutine's wake-up latency,
so a fixed turn count is a wall-clock assumption in disguise. Raising it
would hide the flake, not fix it.

So the budget is gone rather than larger. driveUntilDone drives until
the saving goroutine finishes and nothing else. Termination still holds,
it just belongs to the code under test: every AutoSaveOnSignal returns
within the timeout it is handed, and g.sigSave is one deep, so an
unserviced request stays in the channel and every later call finds it
full and fails at once. A dead handoff therefore releases the saving
goroutine after one autoSaveWait however many saves were asked for, and
what fails is the real assertion - "saves taken = 0, want 25" - rather
than "out of turns".

Removing the cap exposed a second assumption underneath it. testTerm
answers space and newline for ever once its script is exhausted, and
neither key takes a turn, so command(), which loops until the player
consumes one, never returns; the old cap was silently sized to the
script. An uncapped drive wedged inside a single command() call. The two
drive tests now use driveTerm, a headless terminal whose script repeats,
so every key it hands out takes a turn.

The guard is undiminished, shown by mutation and reverted afterwards.
Reverting the fix from the earlier signal-autosave work - AutoSaveOnSignal
replaced by a direct g.autoSave(), encoding on the calling goroutine -
still fails the test with 139 DATA RACE reports, snapshotHeader reading
what executeCommand writes. Removing serviceAutoSaveRequest from
command() still fails it too, now in 10s with "saves taken = 0, want 25"
instead of by hanging.

Under load: at GOMAXPROCS=2 on a 48-core host at load ~150, with an
unrelated deliberate failure in the tree so every run took the verbose
rerun, the old code failed 8 of 8 runs and the new code 0 of 8. Also
green across 24 concurrent unconstrained runs at load ~120, 10 runs
alongside a spinner load, and 5 runs each at GOMAXPROCS 1, 2 and 4.

No non-test code changed. make check green, lint 0 issues, .golangci.yml
byte-identical.

TODO.md

@@ -35,6 +35,63 @@ is finished.
 
 # Completed Steps
 
+- 2026-08-09 `TestAutoSaveOnSignalRacesTurnLoop` de-flaked at the cause
+  (`fix/autosave-turn-budget-36`, closes #36). The failure text was captured
+  before anything was changed and it is **not** a data race: the assertion was
+  `driveUntilDone`'s
+  `t.Fatal("the turn loop ran out of turns before the saves were taken")`, with
+  no `WARNING: DATA RACE` anywhere in the log. The handoff fixed in #24 was
+  working; the test's own drive loop was running out of its fixed 1000-turn
+  budget first.
+
+    Confirmed rather than taken on trust. Instrumenting the loop to report the
+    turns it actually used showed the count tracking scheduling pressure and
+    nothing else: about 60-120 turns at host load ~57 with the whole machine to
+    spread over, 418 at `GOMAXPROCS=4`, 539 and 655 at 2 and 1, and past 1000 —
+    the recorded failure — under the doubled load of the verbose rerun that the
+    test target performs after a failure. The turns between one save being
+    answered and the next request arriving are not work; they are the saving
+    goroutine's wake-up latency, so a fixed turn count is a wall-clock
+    assumption in disguise, which is why raising it would have hidden the flake
+    rather than fixed it.
+
+    So the budget is gone rather than larger. `driveUntilDone` now drives until
+    the saving goroutine finishes and nothing else. Termination is not lost, it
+    just belongs to the code under test instead of to the test: every
+    `AutoSaveOnSignal` returns within the timeout it is handed, and `g.sigSave`
+    is one deep, so an unserviced request stays in the channel and every later
+    call finds it full and fails at once — a dead handoff releases the saving
+    goroutine after one `autoSaveWait` however many saves were asked for, and
+    the failure is then the real assertion (`saves taken = 0, want 25`) instead
+    of "out of turns".
+
+    Removing the cap exposed a second assumption underneath it, which is the
+    reason this is not a one-line diff. `testTerm` answers space and newline for
+    ever once its script is exhausted, and neither key takes a turn, so
+    `command()` — which loops until the player consumes one — never returns; the
+    old cap was silently sized to the script (4000 characters, two per turn,
+    against 1000 turns). An uncapped drive wedged inside a single `command()`
+    call. The two drive tests therefore use a new `driveTerm`, a headless
+    terminal whose script repeats, so every key it hands out takes a turn.
+
+    Both halves of the definition of done were demonstrated by mutation, with
+    the deliberately-broken tree reverted afterwards and `.golangci.yml` left
+    byte-identical (sha256 `021cc83f...46bcb`). Reverting #24 —
+    `AutoSaveOnSignal` replaced by a direct `g.autoSave()`, encoding on the
+    calling goroutine — still fails the test with 139 `WARNING: DATA RACE`
+    reports naming `snapshotHeader` reading what `executeCommand` writes, so the
+    guard is undiminished. Removing the `serviceAutoSaveRequest` call from
+    `command()` still fails it too, now in 10s with `saves taken = 0, want 25`
+    rather than by hanging.
+
+    Under load, an A/B at `GOMAXPROCS=2` on a 48-core host at load ~150, with an
+    unrelated deliberate failure in the tree so that every run took the verbose
+    rerun: the old code failed 8 of 8 runs with "ran out of turns"; the new code
+    failed 0 of 8, the only failure being the planted one. Also green across 24
+    concurrent unconstrained runs at load ~120, 10 runs alongside a spinner
+    load, and 5 runs each at `GOMAXPROCS` 1, 2 and 4. `make check` green, lint 0
+    issues.
+
 - 2026-08-09 Wizard commands under test (`test/wizard-coverage`, closes #7): the
   last of the three thin spots, so the coverage step is now closed rather than
   narrowed. `game/wizard.go`'s eight functions had no tests of their own, and