lib/rt: rename rt_alloc → rt_malloc; rt.alloc → rt.malloc

Hare's canonical runtime allocator is rt::malloc with linker symbol
rt.malloc (ref/hare/rt/malloc.ha:27,78). ww kept the dot→underscore
Plan 9 convention (CLAUDE.md rule 4) so the linker symbol becomes
rt_malloc; the lib/rt exported function name becomes malloc; ww
callers say rt.malloc(...).

The language builtin keyword stays `alloc(T)!` — unchanged from Hare
(ref/hare/hare/lex/token.ha:21 ltok::ALLOC, parse/expr.ha:398
builtin()). The rename only touches the lowered linker symbol and the
exported function name behind it; the user-facing syntax for
heap-allocation is identical to Hare.

Surface:
- rt/alloc.s: TEXT rt_alloc → TEXT rt_malloc, labels updated
- lib/rt/malloc.ww: @symbol("rt_malloc") fn malloc(...) (was rt_alloc/alloc)
- rt/ensure.ww: local FFI decl + call site updated to malloc; `!` dropped
  on the direct FFI call (rt_malloc returns *void, not a tagged union)
- 18 .ww callers: rt.alloc(...) → rt.malloc(...)
- cstage cmd/wcc/check.c + wwstage selfhost/cmd/wcc/check.ww
  alloc-builtin suppression gate routes through ffi_resolve("malloc")
  for the lowering; the user-shadow check still keys on the BUILTIN
  KEYWORD "alloc" since that is what `alloc(...)` parses as. Adding
  "malloc" to the user-shadow check was unnecessary and was reverted
  during pre-commit review.
- cstage cmd/w6c/cgen.c: 2× ffi_resolve("alloc") → ffi_resolve("malloc")
- wwstage cgenexpr/cgenstmt: 2× ffiresolve(c, "alloc") → ffiresolve(c, "malloc")
- Test fixtures (700_e2e, 758_cgalloc_str_field, 990_selfhost, 992_w6l_ww,
  selfhost/test/tagged_ptr_ret.ww): updated inline ww sources to the new
  decl + call form

This is commit 2 of 3 in the lib/rt extraction (#38). Commit 3 closes
the OOM contract — return type becomes nullable *void and the builtin
lowering null-checks + propagates nomem.

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

selfhost/cmd/wcc/mem.ww

@@ -1,7 +1,7 @@
 // selfhost/cmd/wcc/mem.ww — port of cmd/wcc/mem.c.
 //
 // Bump arena allocator. Backed by the runtime page allocator
-// (rt_alloc / rt_free), no libc. Each chunk is mmap'd; when the
+// (rt_malloc / rt_free), no libc. Each chunk is mmap'd; when the
 // current chunk runs out we link a fresh one. Freeing the arena
 // unmaps the chain.
 //
@@ -31,8 +31,8 @@ fn roundup(n: u64, a: u64) u64 = {
 };
 
 export fn newarena() *arena = {
-	let a: *arena = rt.alloc(ARENA_SZ): *arena;
-	a.buf = rt.alloc(INIT_CHUNK): *u8;
+	let a: *arena = rt.malloc(ARENA_SZ): *arena;
+	a.buf = rt.malloc(INIT_CHUNK): *u8;
 	a.off = 0u64;
 	a.cap = INIT_CHUNK;
 	a.next = nil;
@@ -49,14 +49,14 @@ fn grow(a: *arena, need: u64) bool = {
 	if (want > MAX_CHUNK) { want = MAX_CHUNK; };
 	if (want < need) { return false; };	// single allocation too big
 
-	let old: *arena = rt.alloc(ARENA_SZ): *arena;
+	let old: *arena = rt.malloc(ARENA_SZ): *arena;
 	old.buf = a.buf;
 	old.off = a.off;
 	old.cap = a.cap;
 	old.next = a.next;
 	old.total = 0u64;
 
-	a.buf = rt.alloc(want): *u8;
+	a.buf = rt.malloc(want): *u8;
 	a.off = 0u64;
 	a.cap = want;
 	a.next = old;