Migrate the three modules that still carried private
@symbol("rt_alloc") / @symbol("rt_free") bindings onto the public
lib/os.alloc / lib/os.free surface that landed in 87c0883.
memio: 1 alloc (grow) + 2 free (grow's old-buffer drop, dynamicclose).
shlex: 1 alloc (dupstr) + 2 free (freepartial: element strs + slice
header). getopt: 1 alloc (tryparse) + 2 free (tryparse + finish).
ABI identity holds — same rt syms, same shapes, just routed through
the public surface.
rt_ensure stays inline in shlex + getopt; the slice-growth helper
isn't part of os and has no stdlib facade. Comments explain why.
Header rationale comments updated: dropped the now-stale
"lib/io ↔ lib/os C-symbol collision" framing on shlex's inlined
dupstr (that was a pre-#9 concern); reworded shlex's OOM trailer to
match lib/os.ww's documented contract (poisonous pointer, not nil,
fault on deref); fixed memio's dynamicfrom doc to reference
[[os.free]] instead of the retired rt_free name.
980_memio_run / 973_shlex_run / 982_getopt_run all green; bootstrap
byte-identical.
After 12436dd, lib/fmt and lib/log production code routed through
os.write / os.exit. The test files (fmttest, logtest, memiotest)
still carried the same @symbol("rt_syscall") syscall1ww / doexit
stub block with the (now stale) os↔io collision rationale. Same
mechanical drop as 12436dd: add use os;, route fail() through
os.exit, remove the stubs.
Also reword the stale workaround comment in lib/memio/memio.ww
covering rt_alloc/rt_free. The decls themselves stay until lib/os
exports alloc/free as a follow-up.
No combined.ww regen (these files aren't bootstrap-folded).
