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).

examples/cmatrix/cmatrix.ww

@@ -29,6 +29,7 @@
 package cmatrix;
 
 import os;
+import rt;
 import time;
 import fmt;
 
@@ -200,18 +201,18 @@ fn now_ns() u64 = {
 // ---- cmat allocate / free ---------------------------------------------
 
 fn cmat_alloc(w: i32, h: i32, r: *rng) *cmat = {
-	let m = os.alloc(CMAT_SZ): *cmat;
+	let m = rt.alloc(CMAT_SZ): *cmat;
 	m.w = w;
 	m.h = h;
 	m.gspeed = -1;
 	m.gtheme = theme.GREEN: i32;
 	m.flash  = 0;
 	let n4 = (w: u64) * 4u64;
-	m.head    = os.alloc(n4): *i32;
-	m.length  = os.alloc(n4): *i32;
-	m.speed   = os.alloc(n4): *i32;
-	m.counter = os.alloc(n4): *i32;
-	m.glyphs  = os.alloc((w: u64) * (MAX_TRAIL: u64)): *u8;
+	m.head    = rt.alloc(n4): *i32;
+	m.length  = rt.alloc(n4): *i32;
+	m.speed   = rt.alloc(n4): *i32;
+	m.counter = rt.alloc(n4): *i32;
+	m.glyphs  = rt.alloc((w: u64) * (MAX_TRAIL: u64)): *u8;
 	let c = 0;
 	for (c < w) {
 		m.head[c]    = -rng_range(r, 0, h);

examples/lisp/lispcore.ww

@@ -62,6 +62,7 @@
 package lisp;
 
 import os;
+import rt;
 import fmt;
 import ascii;
 import strconv;
@@ -158,7 +159,7 @@ let perm_arena:  arena;
 let trans_arena: arena;
 
 fn arena_new_chunk(prev: *chunk) *chunk = {
-	let raw: *u8 = os.alloc(ARENA_CHUNK): *u8;
+	let raw: *u8 = rt.alloc(ARENA_CHUNK): *u8;
 	let c: *chunk = raw: *chunk;
 	c.next = prev;
 	c.cur  = 0u64;
@@ -1565,10 +1566,10 @@ fn bind_one(e: **env, name: str, id: i32) void = {
 
 export fn initsyms() void = {
 	arena_init();
-	sym_off  = os.alloc((SYM_CAP: u64) * 4u64): *i32;
-	sym_len  = os.alloc((SYM_CAP: u64) * 4u64): *i32;
-	sym_blob = os.alloc(SYM_BLOBSZ: u64): *u8;
-	obuf     = os.alloc(OBUF_SZ: u64): *u8;
+	sym_off  = rt.alloc((SYM_CAP: u64) * 4u64): *i32;
+	sym_len  = rt.alloc((SYM_CAP: u64) * 4u64): *i32;
+	sym_blob = rt.alloc(SYM_BLOBSZ: u64): *u8;
+	obuf     = rt.alloc(OBUF_SZ: u64): *u8;
 
 	// Stash the special-form ids so classify_sform sees them.
 	SID_QUOTE   = intern("quote");