w6c+w6c_ww: loud-stop non-ident/non-local append spread source (#34 review)

A spread whose source was not a local ident fell PAST the spread arm:
cstage continued into the single-value stores with the N_SPREAD node
(garbage store), wwstage silently SKIPPED the value entirely — a silent,
cs!=ww-divergent miscompile (append(ys, f()...): cs exit 0 / ww exit 144,
want 3), reachable for every element kind and predating #34 for scalars.
Both stages now rule-7 loud-stop the shape (deferred, task #37).

Also pins the widener's already-tagged single-value source path
(tagged_ident_src row, i64 + bool members) and adds the
spread_call_loudstop BUILD_FAIL row — test 800 is now 15 rows / 43
fixtures.

test/wcc/800_append_wide_elem.c

@@ -60,10 +60,18 @@
  *                        | scratch slots where wwstage dedups)  |
  *   spread_str           | append(ys, xs...) of []str + single  | 7
  *   spread_tagged        | append(ys, xs...) of [](i64|bool)    | 42
+ *   tagged_ident_src     | append of ALREADY-TAGGED locals      | 42
+ *                        | (i64 + bool members) — the widener's |
+ *                        | already-tagged source path.          |
  *   struct_call_loudstop | append(xs, f()) struct-from-call is  | BUILD_FAIL
  *                        | the deferred source shape — must     |
  *                        | fail LOUD on both stages (rule-7),   |
  *                        | never silently store one word.       |
+ *   spread_call_loudstop | append(ys, f()...) non-ident spread  | BUILD_FAIL
+ *                        | source — pre-review this fell PAST   |
+ *                        | the spread arm (cstage garbage store |
+ *                        | vs wwstage silent skip, divergent);  |
+ *                        | now a rule-7 loud-stop (task #37).   |
  *
  * Every non-BUILD_FAIL row also asserts cstage/wwstage asm byte-id,
  * which subsumes the frame-size canary (TEXT main,$N) — the @tagbase/
@@ -251,6 +259,28 @@ static const struct row rows[] = {
 	  "};\n",
 	  42 },
 
+	{ "tagged_ident_src",
+	  "package main;\n"
+	  "type cell = (i64 | bool);\n"
+	  "export fn main() i32 = {\n"
+	  "\tlet xs: []cell = [];\n"
+	  "\tlet c: cell = 40i64;\n"
+	  "\tlet b: cell = true;\n"
+	  "\tappend(xs, c);\n"
+	  "\tappend(xs, b);\n"
+	  "\tlet r: i32 = 0;\n"
+	  "\tmatch (xs[0]) {\n"
+	  "\tcase let v: i64 => r += v: i32;\n"
+	  "\tcase bool => r = 99;\n"
+	  "\t};\n"
+	  "\tmatch (xs[1]) {\n"
+	  "\tcase i64 => r = 98;\n"
+	  "\tcase let w: bool => { if (w) { r += 2; }; };\n"
+	  "\t};\n"
+	  "\treturn r;\n"
+	  "};\n",
+	  42 },
+
 	{ "struct_call_loudstop",
 	  "package main;\n"
 	  "type pt = struct { x: i32, y: i32, z: i64 };\n"
@@ -263,6 +293,21 @@ static const struct row rows[] = {
 	  "\treturn 0;\n"
 	  "};\n",
 	  BUILD_FAIL },
+
+	{ "spread_call_loudstop",
+	  "package main;\n"
+	  "fn mks() []str = {\n"
+	  "\tlet xs: []str = [];\n"
+	  "\tlet a: str = \"abc\";\n"
+	  "\tappend(xs, a);\n"
+	  "\treturn xs;\n"
+	  "};\n"
+	  "export fn main() i32 = {\n"
+	  "\tlet ys: []str = [];\n"
+	  "\tappend(ys, mks()...);\n"
+	  "\treturn ys[0].len: i32;\n"
+	  "};\n",
+	  BUILD_FAIL },
 };
 
 static int