Netplay rollback: count post-movie agreement in confirmed ticks

The post-FMV lockstep gate counted a tick as agreed only when sim-1 was
already hash-confirmed. A peer digest arrives one RTT after its tick, so
the peer running ahead (the host, cross-machine) never saw sim-1 confirmed,
reset the streak every tick and logged 'rb DESYNC - FMV lockstep MAX
unmatched' after every movie while every digest matched. Each newly
confirmed tick past the movie's end now counts once; only a real mismatch
resets the streak.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

runtime/include/netplay_fmv_confirm.h

@@ -0,0 +1,21 @@
+/* netplay_fmv_confirm.h - post-movie agreement counting for rollback netplay.
+ *
+ * After a movie, both peers must agree for RB_FMV_LOCKSTEP_CONFIRM ticks
+ * before prediction is unlocked. Agreement is measured in hash-confirmed
+ * ticks: each newly confirmed tick past the movie's end counts once, however
+ * late its confirmation arrives. A confirmation is always about one RTT
+ * behind the tick it confirms, so testing "is sim-1 confirmed yet" every tick
+ * never succeeds on the peer that runs ahead. */
+#ifndef PSX_NETPLAY_FMV_CONFIRM_H
+#define PSX_NETPLAY_FMV_CONFIRM_H
+
+#include <stdint.h>
+
+/* Confirmed ticks gained when the confirmed frontier moves from `from` (the
+ * highest tick already counted, or the movie's end) to `resolved`. */
+static inline uint32_t rb_fmv_confirm_gain(uint32_t from, uint32_t resolved)
+{
+    return resolved > from ? resolved - from : 0u;
+}
+
+#endif