lib/fmt+lib/log+lib/memio: drop rt_syscall stubs in tests; use os

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).

lib/memio/memiotest.ww

@@ -8,20 +8,13 @@
 use bytes;
 use io;
 use memio;
-
-// Direct exit(2) binding rather than `use os;` — os exports
-// read/write/close, which collide with io.read/write/close under
-// the driver's flat-scope concat (task #7).
-@symbol("rt_syscall") fn syscall1ww(num: i64, a: i64) i64;
-fn doexit(code: i32) void = {
-	syscall1ww(60i64, code: i64);
-};
+use os;
 
 // signalled — bumped by main before each test so a failing exit code
 // pinpoints the offending case.
 let signalled: i32 = 0;
 
-fn fail() void = { doexit(signalled + 10); };
+fn fail() void = { os.exit(signalled + 10); };
 
 fn putstr(s: str, into: []u8, off: i32) i32 = {
 	let i: i32 = 0;