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/950_selfcheck.c

@@ -144,6 +144,20 @@ static const struct row rows[] = {
 	  "  let s: []u8 = alloc([], 16)!;\n"
 	  "};\n",
 	  NULL },
+	/* #45: alloc([], n) defers element type to LHS context — `[]str`
+	 * with `!` must be accepted (cstage clet retypes, wwstage
+	 * checkletassign mirror). Pre-#45 this errored with
+	 * "init []u8 not assignable to declared []str" on cstage. */
+	{ "fn caller() void = {\n"
+	  "  let s: []str = alloc([], 4)!;\n"
+	  "};\n",
+	  NULL },
+	/* #45: same idiom in `?` form inside a (T | nomem) fn. */
+	{ "fn caller() (i32 | nomem) = {\n"
+	  "  let s: []str = alloc([], 4)?;\n"
+	  "  return s.cap: i32;\n"
+	  "};\n",
+	  NULL },
 };
 
 int