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/ensure.ww

@@ -14,12 +14,12 @@
 // no per-type wrapper functions (appendu8 / appendi64) needed.
 //
 // User code never `use`s this — the symbol is resolved at link time
-// from libwwrt.a, like rt_alloc and rt_streq. No `module` declaration:
+// from libwwrt.a, like rt_malloc and rt_streq. No `module` declaration:
 // rt/ensure.ww is compiled standalone via `w6c rt/ensure.ww` (not
 // through the driver), and its `export fn rt_ensure` must keep its
 // bare symbol name so the linker resolves it.
 
-@symbol("rt_alloc") fn alloc(n: u64) *void;
+@symbol("rt_malloc") fn malloc(n: u64) *void;
 @symbol("rt_free")  fn free(p: *void, n: u64) void;
 
 // Mirrors ww's []T header layout: 24 bytes with 8-byte slots.
@@ -37,12 +37,9 @@ export fn rt_ensure(s: *slice, membsz: u64) void = {
 	let nc: i64 = s.cap * 2i64;
 	if (nc < 8i64) { nc = 8i64; };
 	for (nc < s.len) { nc *= 2i64; };
-	// Task #30: the alloc builtin returns `(*T | nomem)`; `!` aborts
-	// on OOM. The longstanding sizeof-only allocation bug (task #27)
-	// stays unfixed here — rt_ensure presumes a same-module `fn
-	// alloc(n)` resolution that the bare cur_mod=NULL gate at
-	// cmd/wcc/check.c:984 doesn't honour.
-	let np: *u8 = alloc((nc: u64) * membsz)!: *u8;
+	// Task #30 (commit 3) upgrades to nullable *void + null-check.
+	// For now, rt_malloc returns plain *void; OOM faults on deref.
+	let np: *u8 = malloc((nc: u64) * membsz): *u8;
 	let n: u64 = (s.cap: u64) * membsz;
 	let i: u64 = 0u64;
 	for (i < n) {