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/os/os.ww

@@ -12,28 +12,6 @@ import time;
 @symbol("rt_syscall") fn syscall3(num: nr, a: i64, b: i64, c: i64) i64;
 @symbol("rt_syscall") fn syscall4(num: nr, a: i64, b: i64, c: i64, d: i64) i64;
 
-// alloc / free — runtime mmap-backed page allocator. Untyped:
-// `alloc(n)` returns a `*void` and `free(p, n)` requires the byte
-// count back because rt_free is munmap-based and doesn't track
-// mapping sizes (the kernel needs the length to release the
-// reservation).
-//
-// Diverges from Hare. Hare exposes `alloc` / `free` as typed
-// language builtins (`alloc(value, cap)?` / `free(ptr)`) that the
-// compiler lowers to rt::malloc/rt::free; ww has no such builtins,
-// so the rt-symbol surface is exposed directly. Stdlib callers
-// that need a typed allocation pattern wrap this with a cast plus
-// a stored capacity (see [[strings.dup]], [[memio.dynamic]]).
-//
-// OOM: rt_alloc is a bare mmap(MAP_ANON|MAP_PRIVATE) wrapper with
-// no error path. The raw Linux mmap syscall returns a negative
-// errno cast to `*void` on failure (e.g. `(void*)-12` for ENOMEM);
-// the `MAP_FAILED` (`(void*)-1`) value is a libc-wrapper convention
-// that rt_alloc doesn't apply. Neither `== nil` nor `== (void*)-1`
-// catches it; any deref of such a return faults. Today the stdlib
-// does not check; OOM faults on first dereference. A typed
-// fallible variant is a future task.
-@symbol("rt_alloc") export fn alloc(n: u64) *void;
 @symbol("rt_free")  export fn free(p: *void, n: u64) void;
 @symbol("rt_abort") fn abort(msg: str) void;
 

lib/os/ostest.ww

@@ -26,6 +26,7 @@
 package os;
 
 import os;
+import rt;
 
 let signalled: i32 = 0;
 
@@ -102,7 +103,7 @@ fn streq(a: str, b: str) bool = {
 // dereferenceable, not just non-nil).
 
 @test fn test_alloc_free_roundtrip() void = {
-	let p: *u8 = os.alloc(4096u64): *u8;
+	let p: *u8 = rt.alloc(4096u64): *u8;
 	if (p == nil: *u8) { fail(); };
 	// Write a sentinel at the head and tail of the page, read it
 	// back. A miscompiled binding (wrong arg order, wrong ABI, etc.)