wcc+w6c_ww: >48B tagged args from place-resolved sources (#38b cont.)

Task #40 (FB3): the #38b MEMORY-class staging only accepted ident /
aggarg_srcaddr-enumerated sources, loud-stopping on a slice-element
arg — the regex run_thread loop-condition shape
is_consuming_inst(re.insts[threads[i].pc]) (regex.ha:602) and B4's
(*p)[i] spelling. The staging now falls back to cgplaceaddr (the F6
resolver) for the slot's source address; the existing word-push
machinery is unchanged. Placed AFTER aggarg_srcaddr so every
pre-#40 shape keeps its asm byte-exact (verified vs a master w6c).

Boundaries kept loud (rule 7): exact-type rvalue (no place to
resolve), sret-class call source (#40-family follow-up), global
tagged let (task #25), variadic element, register-overflow mixing.
Diagnostic re-worded to match: slice-element is wired, rvalue and
unresolvable-place sources are not.

test/929 grows 6 run rows (element source with literal+computed
index, deref-spine, nested re.insts[threads[i].pc] spelling,
element mixed with register args both orders, fn-call index with
two mem args per call — the inner CALL runs inside the mem
pre-pass with the other slot already staged — and global-slice
element, the storage-backed twin of the loud fail_global_src
boundary — all build-fail loud at master) and 1 reject row pinning
the new boundary text on both stages.

selfhost/cmd/w6c/main.combined.ww

@@ -16000,7 +16000,23 @@ fn pushargsrev(c: *cgen, arg: *node, param: *node, memphase: bool) i32 = {
 			};
 			return rest + memsz / 8;
 		};
-		let mu: str = "#38b: >48B tagged arg from unsupported source kind (slice-element and rvalue sources unwired)\n";
+		// #40/FB3: a place the enumerated arms miss — slice
+		// element, deref-spine element — resolves through the F6
+		// resolver. AFTER aggargsrcaddr so every pre-#40 shape
+		// keeps its asm; the resolver balances its own pushes, so
+		// the words already staged below stay put.
+		if (cgplaceaddr(c, arg, "SI")) {
+			let w: i32 = memsz / 8 - 1;
+			for (w >= 0) {
+				emitline("\tMOVQ\t");
+				emitoff((w*8): i64);
+				emitline("(SI), AX\n");
+				emitline("\tPUSHQ\tAX\n");
+				w -= 1;
+			};
+			return rest + memsz / 8;
+		};
+		let mu: str = "#38b: >48B tagged arg from unsupported source kind (rvalue and unresolvable-place sources unwired)\n";
 		os.write(2, mu.ptr, mu.len: u64);
 		os.exit(1);
 	};

selfhost/cmd/wcc/cgenutil.ww

@@ -214,7 +214,23 @@ fn pushargsrev(c: *cgen, arg: *node, param: *node, memphase: bool) i32 = {
 			};
 			return rest + memsz / 8;
 		};
-		let mu: str = "#38b: >48B tagged arg from unsupported source kind (slice-element and rvalue sources unwired)\n";
+		// #40/FB3: a place the enumerated arms miss — slice
+		// element, deref-spine element — resolves through the F6
+		// resolver. AFTER aggargsrcaddr so every pre-#40 shape
+		// keeps its asm; the resolver balances its own pushes, so
+		// the words already staged below stay put.
+		if (cgplaceaddr(c, arg, "SI")) {
+			let w: i32 = memsz / 8 - 1;
+			for (w >= 0) {
+				emitline("\tMOVQ\t");
+				emitoff((w*8): i64);
+				emitline("(SI), AX\n");
+				emitline("\tPUSHQ\tAX\n");
+				w -= 1;
+			};
+			return rest + memsz / 8;
+		};
+		let mu: str = "#38b: >48B tagged arg from unsupported source kind (rvalue and unresolvable-place sources unwired)\n";
 		os.write(2, mu.ptr, mu.len: u64);
 		os.exit(1);
 	};

selfhost/cmd/wwdump/main.combined.ww

@@ -16000,7 +16000,23 @@ fn pushargsrev(c: *cgen, arg: *node, param: *node, memphase: bool) i32 = {
 			};
 			return rest + memsz / 8;
 		};
-		let mu: str = "#38b: >48B tagged arg from unsupported source kind (slice-element and rvalue sources unwired)\n";
+		// #40/FB3: a place the enumerated arms miss — slice
+		// element, deref-spine element — resolves through the F6
+		// resolver. AFTER aggargsrcaddr so every pre-#40 shape
+		// keeps its asm; the resolver balances its own pushes, so
+		// the words already staged below stay put.
+		if (cgplaceaddr(c, arg, "SI")) {
+			let w: i32 = memsz / 8 - 1;
+			for (w >= 0) {
+				emitline("\tMOVQ\t");
+				emitoff((w*8): i64);
+				emitline("(SI), AX\n");
+				emitline("\tPUSHQ\tAX\n");
+				w -= 1;
+			};
+			return rest + memsz / 8;
+		};
+		let mu: str = "#38b: >48B tagged arg from unsupported source kind (rvalue and unresolvable-place sources unwired)\n";
 		os.write(2, mu.ptr, mu.len: u64);
 		os.exit(1);
 	};