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).

selfhost/cmd/w6c/main.ww

@@ -13,6 +13,7 @@
 package main;
 
 import os;
+import rt;
 import mem;
 import tok;
 import lex;
@@ -61,7 +62,7 @@ fn slurp(path: *u8) (*u8, u64) = {
 	case let e: os.oserror => { os.close(fd); return nil, 0u64; };
 	};
 	let nz: u64 = n: u64;
-	let buf: *u8 = os.alloc(nz + 1u64): *u8;
+	let buf: *u8 = rt.alloc(nz + 1u64): *u8;
 	let rr: (i64 | os.oserror) = os.readall(fd, buf, nz);
 	os.close(fd);
 	let got: i64 = 0i64;