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/memio.ww

@@ -28,9 +28,9 @@
 
 use io;
 
-// Direct rt_alloc/rt_free bindings rather than `use os;` because os
-// exports read/write/close, which collide with io.read/write/close
-// under the driver's flat-scope concatenation. Same pattern as
+// Direct rt_alloc/rt_free bindings — the runtime heap primitives
+// memio.dynamic / dynamicfrom need to own the backing buffer. lib/os
+// holds the same FFI shape but doesn't export it; same pattern as
 // rt/ensure.ww.
 @symbol("rt_alloc") fn rtalloc(n: u64) *void;
 @symbol("rt_free")  fn rtfree(p: *void, n: u64) void;