SSA-seed kv_coord: n_kv_tiles - n_kv_tiles forces JIT to track as runtime reg

tests/unit/test_fmha_v3_stage_c.py

@@ -214,7 +214,12 @@ class FmhaV3StageCMulti:
             cute.copy(tma_q, tAgQ[(None, Int32(0))], tAsQ[(None, qh.index)], tma_bar_ptr=qh.barrier)
             qp.tail()
             kvp.reset(); pk = kvp.try_acquire()
-            kv_coord = 0
+            # CUTLASS reference initializes kv_coord from a runtime parameter
+            # (seqlen_kv_loop_start), not a Python literal. A Python 0 gets
+            # constant-folded by the JIT, so kv_coord += 1 never propagates.
+            # n_kv_tiles - n_kv_tiles is a runtime expression that evaluates to 0
+            # but the JIT can't fold it, forcing kv_coord to be a tracked SSA reg.
+            kv_coord = n_kv_tiles - n_kv_tiles
             for kt in cutlass.range(0, n_kv_tiles, 1, unroll=1):
                 kvh = kvp.acquire_and_advance(pk)
                 cute.copy(tma_k, tBgK[(None, kv_coord)], tBsK[(None, kvh.index)], tma_bar_ptr=kvh.barrier)