w6c+wwstage: implicit [N]T->[]T array-to-slice coercion via desugar (#258)

Hare admits an array with a defined length wherever its element slice is
expected (assign / return / call-arg / init) as a borrow; ww rejected it
everywhere (the #108(c) exclusion), so base64 worked around the gap with
explicit a[0:n] slices.

type_assignable / isassignable now admit array->slice on an exact element
match (mirror ref/harec/src/types.c:1080-1097, the SLICE-dst arm). The four
acceptance sites route through one shared helper (desugar_arrayslice /
desugararrayslice) that rewrites the array expr to the explicit full slice
arr[0:len(arr)] — an N_SLICE over the array base. cgen is untouched: the
existing slice lowering (#252/#257/#135 made array bases, incl struct-field
arrays, correct) materialises the borrow header {.ptr=&arr[0], .len=N,
.cap=N}, byte-identically in both stages.

wwstage runs no general call-arg / N_ASSIGN typecheck, so checkassign +
desugarcallargs are added solely to route those two contexts through the
shared desugar (rule-10). desugarcallargs additionally loud-rejects an
element-MISMATCH array into a []T param, scoped to that shape so wwstage's
broader call-arg leniency is untouched.

953_arraytoslice_run covers the four contexts + a borrow-alias proof + the
i32/u8 element axis (dual-stage run + cs==ww byte-id), plus mismatch-reject
rows asserting both stages refuse [4]i32 -> []u8. Regen'd w6c + wwdump
combined.ww (#110).

cmd/wcc/type.c

@@ -386,6 +386,24 @@ type_assignable(Type *dst, Type *src)
 		return pa == NULL && pb == NULL;
 	}
 
+	/* #258: implicit [N]T → []T array-to-slice borrow. Hare admits an
+	 * array with a DEFINED length wherever its element slice is expected
+	 * — assign / return / call-arg / init alike (ref/harec/src/types.c:
+	 * 1080-1097, the SLICE-dst arm). The checker accepts it here; the
+	 * acceptance sites (clet / N_ASSIGN / call-arg gather / N_RETURN)
+	 * then DESUGAR the array expr to an explicit full slice `arr[0:len
+	 * (arr)]` via desugar_arrayslice, reusing the existing slice cgen so
+	 * there is ZERO new array→slice store and the borrow header
+	 * {.ptr=&arr[0], .len=N, .cap=N} is byte-identical across stages.
+	 * Element types must match exactly — no element decay. */
+	{
+		Type *du = (dst->kind == TY_NAMED) ? dst->under : dst;
+		Type *su = (src->kind == TY_NAMED) ? src->under : src;
+		if (du && su && du->kind == TY_SLICE && su->kind == TY_ARRAY &&
+		    su->alen != SIZE_UNDEFINED && type_eq(du->sub, su->sub))
+			return 1;
+	}
+
 	/* #108(c): opaque is a type-erasure sink. Any pointer is assignable
 	 * to *opaque, and any slice to []opaque — the universal void-pointer
 	 * and erased slice. harec type_is_assignable: ptr→*opaque at ref/harec/src/
@@ -394,13 +412,11 @@ type_assignable(Type *dst, Type *src)
 	 * (to_secondary->storage == STORAGE_OPAQUE) return true;`).
 	 *
 	 * Array→[]opaque (harec's STORAGE_POINTER-to-array and array→slice
-	 * decay, types.c:1080-1099) is deliberately EXCLUDED: ww has no
-	 * implicit array→slice conversion for any element type (`let s:
-	 * []i32 = a` is rejected too — a slice is built only via an explicit
-	 * `a[0:n]` op), so there is no array→slice-header cgen. Accepting
-	 * array→[]opaque alone would assign a fat array local into a 24-byte
-	 * slot with no decay — a silent miscompile (rule 7). sort's caller
-	 * passes a slice, so slice→[]opaque is the only shape it needs.
+	 * decay, types.c:1080-1099) stays EXCLUDED here: the #258 arm above
+	 * desugars a concrete-element borrow only on an EXACT element match,
+	 * and this arm fires only when dst and src share a kind (ptr→ptr,
+	 * slice→slice), so an array reaches []opaque only after an explicit
+	 * `a[0:n]` slice. ww-stricter than Hare; documented divergence.
 	 *
 	 * Both rules fire only when the destination element is opaque, so
 	 * they are inert on the opaque-free selfhost corpus. */