parse,check,type,wwi: @packed struct attribute, both stages (#51)

Hare/harec @packed struct layout: no inter-field or trailing padding, align =
max field align (NOT forced to 1) — matches harec type_store.c + types.c:621
(packed{u8,u64}=size 9/align 8). Parser consumes inline @packed (loud-rejects
unknown struct attrs, both stages); layout gates padding on !packed; cstage
type_eq enforces packed type-distinctness; the .wwi producer round-trips
"struct @packed {". wwstage sets slotsize=size for packed so its composite-ABI
copy matches cstage byte-for-byte. cstage identity is faithful; wwstage identity
rides the deferred #224 nominal-resolvealias arc (#108). Both stages byte-id;
447 tests pass.

cmd/wcc/ww.h

@@ -345,6 +345,8 @@ struct Node {
 	Node		*next;	/* sibling link inside `list` */
 	Node		*attr;	/* @attribute chain (N_ATTR list) */
 	int		export;
+	int		packed;	/* N_TSTRUCT: `struct @packed` — no field/
+				 * trailing padding (harec ast.h:95). */
 	Type		*type;	/* filled in by checker */
 	const char	*tsuffix;	/* typed numeric literal suffix */
 	const char	*module;	/* `// MODULE: foo` directive at the
@@ -487,6 +489,8 @@ struct Type {
 				 * stored as a single 8-byte pointer; null
 				 * is the void variant. Mirrors Hare's
 				 * `(*T | null)` folding. */
+	int		packed;	/* TY_STRUCT laid out with no padding; part
+				 * of type identity (harec types.c:517/621). */
 };
 
 extern Type	*ty_void, *ty_bool, *ty_rune;