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).

test/wcc/994_w6c_ww.c

@@ -202,7 +202,7 @@ main(void)
 		/* #45: alloc-slice `!` form, []T element ≠ u8. */
 		{ "alloc_slice_rune_unw",
 		  "package main;\n"
-		  "import os;\n"
+		  "import rt;\n"
 		  "fn main() i32 = {\n"
 		  "  let s: []rune = alloc([], 8)!;\n"
 		  "  return s.cap;\n"
@@ -211,7 +211,7 @@ main(void)
 		 * enclosing fn's tagged return. */
 		{ "alloc_slice_str_prop",
 		  "package main;\n"
-		  "import os;\n"
+		  "import rt;\n"
 		  "fn doit() (i32 | nomem) = {\n"
 		  "  let s: []str = alloc([], 4)?;\n"
 		  "  return s.cap: i32;\n"