w6c: emit length for string-literal .len (fix #14, align cstage to wwstage)

A string literal is TY_UNTYPED_STR, not TY_STR, so `"abc".len` missed
the typed slice/str pseudo-field gate in cgen.c's N_DOT and fell to the
final base-eval fallback, which left AX=.ptr — `.len` returned the
pointer instead of the length. wwstage's cgdot catch-all already did the
BX->AX shuffle, so the two stages diverged (rule-10). Align cstage UP:
the N_DOT fallback emits MOVQ BX,AX for `.len`. `.ptr` is unchanged
(already returned AX); `.cap` deliberately not added (wwstage catch-all
is ptr/len only — mirror exactly).

byte-id was blind here: no bootstrap source uses literal `.len` (lengths
are hardcoded around literals), so the gate never exercised it. New test
801 pins both dimensions (cstage run + cs==ww byte-id) over
len/empty/multibyte/ptr-deref/arg-passthrough rows.

selfhost/cmd/wwdump/main.combined.ww

@@ -22616,10 +22616,11 @@ fn cgdot(c: *cgen, n: *node) void = {
 		};
 	};
 	// Non-ident base pseudo-field: e.g. `"abc".ptr` / `"abc".len`.
-	// Evaluate the str-producing expression — that leaves
-	// (AX=ptr, BX=len). Then `.ptr` returns AX as is; `.len`
-	// shuffles BX→AX. Mirrors what C cgen does (it just evaluates
-	// the literal and picks the half it wants).
+	// A string literal is TY_UNTYPED_STR, so it misses the typed
+	// slice/str gate above and lands here. Evaluate the str-producing
+	// expression — that leaves (AX=ptr, BX=len). Then `.ptr` returns
+	// AX as is; `.len` shuffles BX→AX. cstage cgen.c was aligned UP
+	// to this shuffle in #14 (it had returned the ptr for `.len`).
 	if (streq(fld, "ptr")) { cgexpr(c, lhs); return; };
 	if (streq(fld, "len")) {
 		cgexpr(c, lhs);