feat: race over Futures via context.io.race (PLAN-IO-UNIFY Phase 4)

Re-home the proven first-wins race from sched.race(*Task) onto *Future handles
+ the Io protocol; the old Task-based race is REPLACED (ufcs overload-by-receiver
is rejected, and only 1821 used it).

- Protocol: add Io.current_park() -> ParkToken — the running fiber as a token,
  captured WITHOUT parking — so race can register the SAME coordinator across N
  futures' park slots, then park once via suspend_raw; any completion readies it.
  Scheduler returns {self.current} (bails outside a fiber); CBlockingIo returns
  {null} (race never parks there — futures are born .ready).
- race :: ufcs (io: Io, futures: $T) -> RaceResult(T), kept in sched.sx (it needs
  meta.sx's make_enum/make_variant; pulling that into the io.sx prelude part-file
  would cycle). Winner scan -> register/park/deregister -> make_variant the winner
  -> Phase-3 cancel each still-.pending loser (no join). RaceResult reused
  unchanged (*Future(R) projects field 0 'value' -> R).
- TRUE-cancel: parked losers stop at their next suspend (timers evicted by cancel's
  wake), so race returns at WINNER-time, not slowest-loser-time.
- Adversarial review fixes: (1) an all-failing/all-cancelling racer set no longer
  deadlock-aborts the scheduler — race bails loudly ('all futures settled without
  a winner') when nothing is .ready and nothing is still .pending; (2) only
  .pending losers are cancelled, so a loser that already .failed keeps its real
  outcome label instead of being stomped to .canceled.

Re-point 1821 to context.io.async + context.io.race (winner a=111, losers
.canceled, completion log only 'task 1 @ 10ms', final clock 10ms — was 30 under
the old cooperative join). New 1826 locks the failing-loser case. Byte-identical
on aarch64-macOS + aarch64-linux. Suite 853/0; .ir churn is the current_park
vtable method.

examples/concurrency/expected/1821-concurrency-fiber-race.stdout

@@ -1,8 +1,6 @@
 winner: a (i64)  = 111
-loser b: canceled=1 finished=1
-loser c: canceled=1 finished=1
+loser b: canceled=true
+loser c: canceled=true
 completion order (id @ virtual-ms):
   task 1 @ 10ms
-  task 2 @ 20ms
-  task 3 @ 30ms
-final virtual clock: 30ms
+final virtual clock: 10ms

examples/concurrency/expected/1826-concurrency-fiber-race-failing-loser.build

@@ -0,0 +1 @@
+{ "target": "macos" }

examples/concurrency/expected/1826-concurrency-fiber-race-failing-loser.exit

@@ -0,0 +1 @@
+0

examples/concurrency/expected/1826-concurrency-fiber-race-failing-loser.stderr

@@ -0,0 +1 @@
+

examples/concurrency/expected/1826-concurrency-fiber-race-failing-loser.stdout

@@ -0,0 +1,3 @@
+winner: b = 42
+a: failed=true canceled=false
+final clock: 10ms