cgen: store all eightbytes when an in-cap aggregate call returns into a field of an indexed element (#11)

The arr[i].f=mk() assign arm had no aggregate-field sub-arm, so a by-value aggregate field receive fell to the scalar default (one MOVQ, dropping DX/CX) — silent on BOTH stages (byte-id blind). Add a dual-site symmetric in-cap N_CALL arm mirroring C2c (c83a340): scratch-first materialise AX/DX/CX, then word-copy to (fi.foff+k*8) within &arr[i], sizing from the natural field size fi.fsz (not slotsize). Rule-7 LOUD-STOP for the three cases the in-cap GP path cannot transport: over-cap sret (#11c/#234), a float-bearing field whose eightbyte classifies SSE (#11/#165), and a 3/5/6/7-byte sub-8 tail the single narrow tail MOV cannot express (the general cascade tail is the shared C2c/#11 follow-up, task #10). Value-asserting pins (poison-seeded, redden under each stage's independent revert) plus cfail pins for the three loud-stops.

Contained to the indexed base + in-cap call rhs; arr[i].f=src (#11b) and over-cap (#11c) are separate.

test/wcc/data/idx_dot_aggret_overcap_loud/case.ww

@@ -0,0 +1,15 @@
+//ww:error "over-cap (sret) aggregate field receive"
+// #11c/#234 tripwire: an OVER-cap (>24B, sret-returning) CALL into an aggregate
+// field of an indexed element. The result is written via a dest pointer, not the
+// AX/DX/CX cursor, so the in-cap materialise cannot handle it -> both stages
+// LOUD-STOP until the const-idx sret-into-field path is wired (task #8, rule 7).
+// SUCCESS = the over-cap stop regressed to a 1-word silent drop (cs!=ww).
+package main;
+type big = struct { a:i64, b:i64, c:i64, d:i64 };
+type s = struct { f: big, g: i64 };
+fn mk() big = { return big{a=1i64,b=2i64,c=3i64,d=4i64}; };
+export fn main() i32 = {
+	let arr: [2]s;
+	arr[1].f = mk();
+	return 0;
+};