w6c+wwstage: emit over-cap tuple return via sret callee-side (#10 Fold A)

A tuple return whose SysV register-return footprint exceeds the caps
(> 4 integer eightbytes or > 2 SSE eightbytes) previously LOUD-STOPPED
at the N_RETURN SEND. Fold A makes the CALLEE emit such a return through
the existing >24B-struct sret skeleton:

  - classifier (cg_sret_retsize / sretretsize) grows a TY_TUPLE arm:
    walk the element footprint over the SAME caps the SEND uses, and
    return the tuple's natural total size (type table) when over-cap,
    else 0. The gp/sse caps are factored to a single shared SSoT
    (TUPLE_GPCAP / TUPLE_SSECAP — cgen.c macros in cstage, cgen.ww defs
    in wwstage) consumed by the classifier AND every emit/receive site
    (the SEND, the destructure guards, the cgcall arg guard) — so
    classify and emit can't disagree in either stage.
  - the SEND replaces the loud-stop with a write-through: cgexpr each
    element, store it through *(@sretarg) at its packed layout offset
    (the t.0/t.1 positional layout), each at its natural width so a
    narrow tail stores MOVL/MOVB not an over-MOVQ (#169); the dest base
    reloads into DX each step since a wide element clobbers AX/BX/CX.
    Then the existing struct-sret epilogue (MOVQ @sretarg->AX; ret).
  - the prologue already wires @sretarg when the classifier is nonzero.

The CALL/receive side is deliberately untouched: the N_MLET/N_MASSIGN
destructure loud-stops stay, so an over-cap tuple return is not yet
usefully callable. The end-to-end round-trip arrives with Fold B (#10-B).

Symmetric cstage (cmd/w6c/cgen.c) + wwstage (cgen.ww / cgenstmt.ww /
cgenutil.ww); combined.ww amalgams regenerated. Test 798 asserts the
callee now COMPILES (no loud-stop) and w6c vs w6c_ww .s byte-identical
across all-wide, str, narrow-tail, and float-over-cap shapes; no runtime
row (uncallable until Fold B). All 236 pass incl. 990-997 byte-id.

selfhost/cmd/wcc/cgen.ww

@@ -449,6 +449,15 @@ type enumtype = struct {
 def LOOP_MAX: i32 = 16;
 def DEFER_MAX: i32 = 16;
 
+// The SysV register-return-ABI caps — the SINGLE SSoT shared by the sret
+// classifier (sretretsize over-cap-tuple arm) AND every emit/receive site
+// (cgreturn tuple SEND, cgmlet/cgmassign destructure, cgcall arg guard).
+// Classify and emit MUST agree on these, else a tuple gets classified
+// sret by one and in-reg by the other -> corruption. Mirrors cstage
+// cgen.c TUPLE_GPCAP/TUPLE_SSECAP (#10).
+def TUPLE_GPCAP: i32 = 4;	// AX,DX,CX,R8
+def TUPLE_SSECAP: i32 = 2;	// X0,X1
+
 type cgen = struct {
 	locals:   *local,
 	// atlocals — persistent registry of `@`-prefix scratch slots