test/684: add 1-element [_] edge + module element read-back rows

The reviewer flagged two gaps in the #7 coverage: no minimal
non-empty count (a 1-element [_] is the boundary the element-counter
must still get right) and no module-level array element read-back
(only .len was checked at module scope). Add local_one_len /
mod_one_len / mod_one_elem. Both still mutation-resistant: the old
collapse-to-0 reads .len as 0, not 1, so the new len rows fail it too.
42/42 dual-stage + byte-id.

test/wcc/684_arr_infer_len.c

@@ -41,6 +41,9 @@
  *   mod_str_len     | global [_]str=["a","b","c"], x.len   | 3
  *   local_u8_len    | local [_]u8=[1..5], x.len            | 5
  *   local_u8_elem   | local, x[4]                           | 5
+ *   local_one_len   | local [_]int=[7], x.len (1-elem edge)| 1
+ *   mod_one_len     | global [_]int=[7], x.len             | 1
+ *   mod_one_elem    | global, x[0]                          | 7
  */
 #include <stdio.h>
 #include <stdlib.h>
@@ -132,6 +135,33 @@ static const struct row rows[] = {
 	  "\treturn x[4]: i32;\n"
 	  "};\n",
 	  5 },
+
+	/* 1-element edge — the minimal non-empty count; under the old
+	 * collapse-to-0 bug `.len` reads 0, not 1, so this row also fails
+	 * the mutation. */
+	{ "local_one_len",
+	  "package main;\n"
+	  "export fn main() i32 = {\n"
+	  "\tlet x: [_]int = [7];\n"
+	  "\treturn x.len: i32;\n"
+	  "};\n",
+	  1 },
+
+	{ "mod_one_len",
+	  "package main;\n"
+	  "let x: [_]int = [7];\n"
+	  "export fn main() i32 = {\n"
+	  "\treturn x.len: i32;\n"
+	  "};\n",
+	  1 },
+
+	{ "mod_one_elem",
+	  "package main;\n"
+	  "let x: [_]int = [7];\n"
+	  "export fn main() i32 = {\n"
+	  "\treturn x[0]: i32;\n"
+	  "};\n",
+	  7 },
 };
 
 /* `[_]T` that can't infer its length — both stages must FAIL the build