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.

rt/alloc.s

@@ -1,6 +1,6 @@
 // rt/alloc.s — page allocator via the mmap syscall.
 //
-// rt_alloc(n: u64) returns a *void aligned at a page boundary, sized
+// rt_malloc(n: u64) returns a *void aligned at a page boundary, sized
 // to the next page multiple. Pair with rt_free(p, n).
 //
 // We pin to PROT_READ|PROT_WRITE and MAP_PRIVATE|MAP_ANONYMOUS so
@@ -12,7 +12,7 @@
 // the failure path here returns 0 instead of a poisoned pointer. The
 // builtin's caller is expected to `!`/`?` the result.
 
-TEXT rt_alloc,$0
+TEXT rt_malloc,$0
 	MOVQ	DI, SI		// arg 1: length = caller's n
 	MOVQ	$0, DI		// arg 0: addr = NULL (kernel chooses)
 	MOVQ	$3, DX		// arg 2: prot = R|W
@@ -22,9 +22,9 @@ TEXT rt_alloc,$0
 	MOVQ	$9, AX		// syscall: mmap
 	SYSCALL
 	CMPQ	$0, AX
-	JGE	rt_alloc_ok
+	JGE	rt_malloc_ok
 	XORQ	AX, AX
-rt_alloc_ok:
+rt_malloc_ok:
 	RET
 
 TEXT rt_free,$0