fibers: Scheduler.deinit + struct-literal init cleanup

Scheduler.deinit closes the bounded leaks B1 documented: it reaps any leftover
ready fibers, frees every heap Task from go (now tracked via a task_allocs
field), frees the timers/io_waiters/task_allocs List backings, and closes the
lazily-opened kqueue fd. Terminal + idempotent; the per-spawn/go closure env
remains unfreeable (language limitation). Locked by
examples/concurrency/1820-concurrency-fiber-scheduler-deinit.sx, which exercises
every freed resource under a tracking GPA (freed by deinit: 5, kq reset to -1).

Also converts plain-struct '= ---'+field-assign init to '.{ ... }' literal init
where '---' carries no meaning: Scheduler.init, Dock.make, and the fiber
examples 1811/1813/1814/1816 (partial literals zero-fill the index-filled array
fields). Unions, '---'-feature tests, the 0154 regression, documented
generic-pack gaps, and loop/conditional inits are intentionally left on '---'.

examples/concurrency/expected/1820-concurrency-fiber-scheduler-deinit.build

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

examples/concurrency/expected/1820-concurrency-fiber-scheduler-deinit.exit

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

examples/concurrency/expected/1820-concurrency-fiber-scheduler-deinit.stderr

@@ -0,0 +1 @@
+

examples/concurrency/expected/1820-concurrency-fiber-scheduler-deinit.stdout

@@ -0,0 +1,5 @@
+read: 3 [97 98 99]
+freed by deinit: 5
+live after deinit: 5
+kq open after run: true
+kq after deinit: -1