Port library/modules/std/sched.sx to run on aarch64-linux alongside
aarch64-macOS, validated byte-identical on both via Apple `container`.
Per-OS bits are comptime-branched:
- MAP_AP (mmap MAP_ANON flag): linux 0x22 / macOS 0x1002.
- fd-readiness backend: epoll on linux, kqueue on darwin (epoll import
scoped to the linux branch). block_on_fd, the run-loop Mode-2 drain,
and cancel_io_waiter_for each branch; the epoll paths EPOLL_CTL_DEL on
fire and on early-wake (EPOLLONESHOT only disables a registration;
kqueue EV_ONESHOT auto-removes it).
- first-entry trampoline: a per-OS hand-written global-asm symbol becomes
a naked sx fn fib_tramp (mov x0,x19; br x20) + register-indirect
dispatch (spawn presets regs[1] == x20 == &fib_dispatch), dropping the
per-OS .global symbol entirely.
Fixes issue 0193 Bug A: the trampoline redesign bus-errored on the
go/wait/sleep capstone (1817) until `export "fib_dispatch"` was restored.
Without the export, fib_dispatch reverts to sx's internal ABI (x0 =
implicit context, first arg self shifted to x1) while the trampoline
hands self over in x0 (C-ABI); on first entry the body runs (x1 happens
to alias self) but the closure then loads regs[1] == &fib_dispatch as its
first capture and re-invokes fib_dispatch forever -> stack overflow ->
bus error. The export pins fib_dispatch to the C-ABI (self in x0),
matching the trampoline. Root cause found via lldb on an AOT build;
confirmed against the compiler source.
Bug B (a top-level asm block wrapped in inline-if is dropped during the
comptime-conditional flatten) is carved out to issue 0194 (OPEN) -- no
live trigger remains, since the naked-fn trampoline sidesteps it.
1811/1814/1816/1817 run byte-identical on the aarch64-macOS host and in
an aarch64-linux container; full suite green (817/0). Documents the fiber
runtime in readme.md.
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