cmd+selfhost+test: relax alloc-slice element-type pin via LHS retype

`alloc([], n)` synthesizes ([]u8 | nomem) at expression level — that's
fine, since the slice form only legitimately appears in let-init
position where the LHS carries the real element type. In clet, after
type-checking the rhs, peel any N_TRYPROP/N_TRYUNW wrapper, match the
alloc-slice AST shape with the same-module shadow gate (from #23),
and retype the call's tagged return to ([]T | nomem) where T is the
declared LHS element. Then assignability sees []T vs []T and accepts.

Cgen N_LET shortcut gains a viatryprop arm next to the existing
viatryunw — on rt_alloc returning null, emits the tagged-return
nomem propagation (MOVQ $nidx, AX; epilogue) instead of exit(1).
nidx comes from cg_tag_for_variant on the enclosing fn's return type,
matching the existing TRYPROP propret path.

Wwstage mirrors all four hunks (check.ww + cgenstmt.ww). Promotes the
previously-silent conf=false skip into a confident accept.

Unblocks #6 (dupall) and lays the path for #4/#7. Byte-identity
holds modulo the pre-existing #44 alloc/rt_alloc symbol divergence.

test/wcc/994_w6c_ww.c

@@ -199,6 +199,24 @@ main(void)
 		  "  for (let (k, v) .. s) { total += k + v; };\n"
 		  "  return total: i32;\n"
 		  "};" },
+		/* #45: alloc-slice `!` form, []T element ≠ u8. */
+		{ "alloc_slice_rune_unw",
+		  "package main;\n"
+		  "import os;\n"
+		  "fn main() i32 = {\n"
+		  "  let s: []rune = alloc([], 8)!;\n"
+		  "  return s.cap;\n"
+		  "};" },
+		/* #45: alloc-slice `?` form propagates nomem via the
+		 * enclosing fn's tagged return. */
+		{ "alloc_slice_str_prop",
+		  "package main;\n"
+		  "import os;\n"
+		  "fn doit() (i32 | nomem) = {\n"
+		  "  let s: []str = alloc([], 4)?;\n"
+		  "  return s.cap: i32;\n"
+		  "};\n"
+		  "fn main() i32 = { return doit()!; };" },
 		{ NULL, NULL },
 	};