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 '---'.
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