test: pin i64-sink chained global-ptr read is full-width, guards #18

#18 verified there is no spurious load-narrow on a chained global-ptr field
read into an i64 sink -- the only MOVSXD is the legitimate :i32 return cast,
byte-identical in both stages. Lock it: a chain_i64_sink row reads q:i64 =
0x1_0000_0001 and asserts the high word == 1. A truncating load-narrow would
drop the high word to 0, so the row pins the spine-narrow family (runtime +
byte-id, both stages).

test/wcc/689_globptr_field_read_run.c

@@ -31,6 +31,7 @@
  *   bump param p.f (param control) | 10
  *   gp.sf.len (CHAINED str leaf)   | 3   (#16)
  *   gp.x.q    (CHAINED struct leaf)| 7   (#16)
+ *   gp.x.q i64 sink, (v>>32)       | 1   (#18 spine-narrow PIN)
  * Each row also asserts w6c .s == w6c_ww .s byte-identical.
  * Pre-fix: cstage SEGV(139) on every global-ptr row; wwstage ran wrong;
  * cs vs ww .s differed.
@@ -153,6 +154,20 @@ static const struct row ROWS[] = {
 	"let gp: *S = nil;\n"
 	"export fn main() i32 = { let s: S = S{a=0,x=Inner{p=0,q=0}}; gp = &s;"
 	" s.x.q = 7; return gp.x.q: i32; };\n", 7 },
+/* #18 PIN: a CHAINED read into an i64 SINK must NOT carry a spurious load/
+ * spine narrow. q holds 0x1_0000_0001; we observe the HIGH word via
+ * (v>>32):i32 — a truncating MOVSXD on the chained load would sign-extend
+ * the low word (1) and the high word would vanish (1 -> 0). The legitimate
+ * MOVSXD must stay on the `:i32` RETURN cast only (after a full 64-bit
+ * SARQ over the complete i64). Verified no-bug at HEAD; this row guards the
+ * spine-narrow family (cf. fld-load-op / int-cast-no-truncate fixes). */
+{ "chain_i64_sink",
+	"package main;\n"
+	"type T = struct { q: i64 };\n"
+	"type S = struct { x: T };\n"
+	"let gp: *S = nil;\n"
+	"export fn main() i32 = { let s: S = S{x=T{q=0}}; gp = &s;"
+	" s.x.q = 4294967297; let v: i64 = gp.x.q; return (v >> 32): i32; };\n", 1 },
 };
 
 #define NROWS ((int)(sizeof ROWS / sizeof ROWS[0]))