lib/crypto/sha256: cite #269 for the def-array-dim divergence (rule-7)

The literal array dimensions ([64]u8/[8]u32/[64]u32) where Hare uses the
BLOCKSZ def / [_]u32 are forced by ww rejecting a def in array-dimension
position. rule-7 requires a retained divergence carry a filed-task
pointer; add the #269 cite to the header divergence list and the state.x
at-site note (previously described the limitation but cited no task).

lib/crypto/sha256/sha256.ww

@@ -18,6 +18,10 @@
 //     [[sha256]] rather than pointing at a module-level const vtable
 //     (the base64/memio/io convention; ww has no const-vtable-of-fn-ptrs
 //     idiom).
+//   - array dimensions are integer literals (`[64]u8`, `[8]u32`,
+//     `[64]u32`) where Hare uses the `BLOCKSZ` def / `[_]u32`: ww rejects
+//     a def in array-dimension position ("array length must be an integer
+//     literal"). Retained language divergence — filed as #269.
 //   - slice-to-slice copy assignment (`h.x[a..b] = b[..n]`) is spelled
 //     as a byte loop — ww has no slice-copy assignment (cf lib/memio's
 //     flat-field note). Same observable effect.
@@ -78,7 +82,7 @@ def k: [64]u32 = [
 export type state = struct {
 	hash.hash,
 	h:  [8]u32,
-	x:  [64]u8,	// BLOCKSZ; array dims require an integer literal
+	x:  [64]u8,	// BLOCKSZ; def array dims rejected (#269)
 	nx: size,
 	ln: size,
 };