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/w6a/asm.ww

@@ -23,7 +23,7 @@ export fn emitbyte(a: *asm_, b: u8) void = {
 		let nc: u64 = a.textcap;
 		if (nc == 0u64) { nc = 4096u64; };
 		nc = nc * 2u64;
-		let nb: *u8 = rt.alloc(nc): *u8;
+		let nb: *u8 = rt.malloc(nc): *u8;
 		let i: u64 = 0u64;
 		for (i < a.textlen) { nb[i] = a.text[i]; i += 1u64; };
 		a.text = nb;
@@ -60,7 +60,7 @@ export fn emitdatabyte(a: *asm_, b: u8) void = {
 		let nc: u64 = a.datacap;
 		if (nc == 0u64) { nc = 256u64; };
 		nc = nc * 2u64;
-		let nb: *u8 = rt.alloc(nc): *u8;
+		let nb: *u8 = rt.malloc(nc): *u8;
 		let i: u64 = 0u64;
 		for (i < a.datalen) { nb[i] = a.data[i]; i += 1u64; };
 		a.data = nb;