fibers: deterministic virtual-time timers (B1.4b)

Add a virtual clock + sleep timers to the M:1 scheduler so fibers
schedule in reproducible simulated time. Scheduler gains clock_ms (the
virtual clock, advances only as timers fire), a timers list, now_ms(),
sleep(ms) (arm {clock_ms+ms, current} + suspend), and a timer-driven
run (drain ready -> fire earliest timer -> advance clock -> wake ->
repeat; the orphan-suspend deadlock check is preserved for a genuine
no-timer park). Wakes fire in deadline order with a FIFO tiebreak.

Adversarial review found a use-after-free: a fiber woken early (manual
or Task wake) before its sleep timer fired was reaped while its Timer
kept a dangling *Fiber, so a later fire dereferenced freed memory.
Fixed: wake evicts the fiber's pending timer (cancel_timer_for) -- every
re-ready path funnels through wake, so no stale timer outlives its fiber.

Examples: 1814 (sim-timer deadline ordering), 1815 (early-wake timer
eviction regression). Suite green 753/0.

examples/concurrency/expected/1814-concurrency-fiber-sim-timer.build

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

examples/concurrency/expected/1814-concurrency-fiber-sim-timer.exit

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

examples/concurrency/expected/1814-concurrency-fiber-sim-timer.stderr

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+

examples/concurrency/expected/1814-concurrency-fiber-sim-timer.stdout

@@ -0,0 +1,8 @@
+wake order (id @ virtual-ms):
+  id=2 @ 10ms
+  id=4 @ 15ms
+  id=5 @ 15ms
+  id=3 @ 20ms
+  id=1 @ 30ms
+final virtual clock: 30ms
+spawned: 5

examples/concurrency/expected/1815-concurrency-fiber-timer-early-wake.build

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

examples/concurrency/expected/1815-concurrency-fiber-timer-early-wake.exit

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

examples/concurrency/expected/1815-concurrency-fiber-timer-early-wake.stderr

@@ -0,0 +1 @@
+

examples/concurrency/expected/1815-concurrency-fiber-timer-early-wake.stdout

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+log: 2 1
+clock: 0  n_suspended: 0