lib: extract rt module from os, sweep imports

Hare puts runtime allocation in rt::, not os:: (ref/hare/rt/malloc.ha:27,
README). ww's `@symbol("rt_alloc") fn alloc(n: u64) *void;` lived at
lib/os/os.ww as a historical bootstrap shortcut; this commit relocates
it to a new lib/rt/malloc.ww and sweeps every site that depended on
`import os` for the alloc decl over to `import rt`.

This is commit 1 of 3 in the lib/rt extraction (#35):
  1. (this) move decl, sweep imports — preserves shape
  2. rename rt_alloc → rt_malloc (#38)
  3. nullable return type + OOM-propagating builtin lowering (#39)

No rename here. Symbol stays rt_alloc, function stays `alloc`, return
stays *void. Behavior identical — same ffi resolution outcome, just
sourced from a different module file. The rt::ensure runtime helper at
selfhost/rt/ensure.ww is its own compilation unit with a local decl and
is untouched.

Side effect: every wcc cgen file used `rt` as a local *node variable
name for "return type." `import rt;` shadows the module, so each
selfhost/cmd/wcc/{check,cgenstmt,cgenexpr,cgenutil}.ww site renamed
to `rtyp`. Mechanical follow-through; only the wcc module-import was
forced to do this rename.

Verified 132/132 + 995_self_rebuild byte-identity (5 wwstage tools
round-trip byte-identical).

lib/fmt/fmt.ww

@@ -29,6 +29,7 @@ package fmt;
 import io;
 import memio;
 import os;
+import rt;
 import strconv;
 
 // i64dec_buf — scratch buffer for [[i64dec]] below. Module-level
@@ -885,7 +886,7 @@ export fn asprintf(fmt: str, args: field...) str = {
 		match (cres) { case void => {}; case io.closed => {}; };
 		return out;
 	};
-	let tight: *u8 = os.alloc(view.len: u64): *u8;
+	let tight: *u8 = rt.alloc(view.len: u64): *u8;
 	let i: i32 = 0;
 	for (i < view.len) {
 		tight[i] = view.ptr[i];