Add os.alloc(n: u64) *void and os.free(p: *void, n: u64) void as
`export fn` via @symbol("rt_alloc") / @symbol("rt_free"). Signatures
mirror lib/memio's existing internal bindings byte-for-byte — only
the name and `export` keyword change. lib/memio + lib/shlex + lib/
getopt drop their own copies in a follow-up commit.
Doc comment spells out the actual failure ABI: rt_alloc wraps the
raw mmap syscall (no libc), so OOM yields a negative-errno cast to
`*void` (e.g. (void*)-12 for ENOMEM). Neither `== nil` nor the libc
MAP_FAILED `(void*)-1` value catches it; deref faults. A typed
fallible variant is future work (alongside #16 fmt.asprintf).
Test (ostest test_alloc_free_roundtrip, signalled=5): alloc 4096B,
write 0x5a at head + 0xa5 at tail, read-back asserts both, free.
The head+tail write/read prevents DCE (failure path calls os.exit)
and proves a real page is backing the returned pointer.
Hojun-Cho
87c088359d