Fix seeking: snapshot timing, backwards time, entity cleanup

Server:
- Reset nextSnapshotTime for active clients after seek so
  SV_SendClientMessages doesn't skip sending (was comparing
  future nextSnapshotTime against past svs.time)
- First frame always a keyframe (beginning seekable)
- Keyframes during normal playback only reset delta state,
  no SNAPFLAG_RESET_ENTITIES (no visual glitch every 5 sec)

Engine client:
- Reset cl.oldFrameServerTime on SERVERCOUNT toggle to prevent
  "time went backwards" error in CL_SetCGameTime

cgame:
- Handle backwards time in CG_ProcessSnapshots for demo seek:
  accept the time jump, reset cg.time, clear all entity state
  (currentValid, interpolate, time-dependent fields), clear
  local entities (particles, gibs, smoke), wait for next snapshot
- Prevents CG_InterpolateEntityPosition NULL nextSnap error

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

code/cgame/cg_snapshot.c

@@ -402,8 +402,34 @@ void CG_ProcessSnapshots( void ) {
 			CG_SetNextSnap( snap );
 
 
-			// if time went backwards, we have a level restart
+			// if time went backwards, we have a level restart or demo seek
 			if ( cg.nextSnap->serverTime < cg.snap->serverTime ) {
+				if ( cg.svDemoPlayback ) {
+					// demo seek — discard old snap, use nextSnap as current,
+					// and wait for another snapshot before rendering
+					cg.snap = cg.nextSnap;
+					cg.nextSnap = NULL;
+					cg.time = cg.snap->serverTime;
+					// reset all entity state and time-dependent fields
+					{
+						int e;
+						for ( e = 0; e < MAX_GENTITIES; e++ ) {
+							cg_entities[e].currentValid = qfalse;
+							cg_entities[e].interpolate = qfalse;
+							cg_entities[e].muzzleFlashTime = 0;
+							cg_entities[e].trailTime = 0;
+							cg_entities[e].dustTrailTime = 0;
+							cg_entities[e].miscTime = 0;
+							cg_entities[e].snapShotTime = 0;
+							cg_entities[e].previousEvent = 0;
+							cg_entities[e].teleportFlag = 0;
+						}
+					}
+					// clear local entities (particles, gibs, etc.)
+					// they reference old times and would render incorrectly
+					CG_InitLocalEntities();
+					break;  // exit loop, wait for next snapshot
+				}
 				CG_Error( "CG_ProcessSnapshots: Server time went backwards" );
 			}
 		}

code/client/cl_parse.c

@@ -308,6 +308,7 @@ void CL_ParseSnapshot( msg_t *msg ) {
 			cl.serverTimeDelta = cl.snap.serverTime - cls.realtime;
 			cl.oldServerTime = cl.snap.serverTime;
 			cl.serverTime = cl.snap.serverTime;
+			cl.oldFrameServerTime = cl.snap.serverTime;  // prevent backwards time error on seek
 		}
 	}
 

code/server/sv_netdemo.c

@@ -396,7 +396,8 @@ static qboolean SVD_StartRecording( const char *demoname ) {
 		} else {
 			demo.keyframeInterval = 0;
 		}
-		demo.framesSinceKeyframe = 0;
+		// first frame is always a keyframe (makes beginning seekable)
+		demo.framesSinceKeyframe = demo.keyframeInterval;
 		demo.numKeyframes = 0;
 	}
 
@@ -1083,13 +1084,24 @@ void SVD_Seek_f( void ) {
 	// toggle SERVERCOUNT to reset client time delta
 	svs.snapFlagServerBit ^= SNAPFLAG_SERVERCOUNT;
 
-	// mark that we seeked — next SVD_ReadFrame sets SNAPFLAG_RESET_ENTITIES
 	demo.seeked = qtrue;
-	// on the next SV_Frame. SVD_ReadFrame will detect the keyframe flag
-	// and set SNAPFLAG_RESET_ENTITIES + reset delta state.
 	demo.endOfDemo = qfalse;
 
-	Com_Printf( "Seeked to keyframe at time %d.\n", demo.keyframeTimes[bestKf] );
+	// reset client snapshot timing so SV_SendClientMessages doesn't
+	// skip sending (nextSnapshotTime was in the future, now svs.time is past)
+	{
+		int j;
+		for ( j = 0; j < sv_maxclients->integer; j++ ) {
+			if ( svs.clients[j].state >= CS_ACTIVE ) {
+				svs.clients[j].nextSnapshotTime = svs.time;
+			}
+		}
+	}
+
+	// ensure one frame runs on next SV_Frame
+	sv.timeResidual = 1000 / sv_fps->integer;
+
+	Com_Printf( "Seeked to time %d.\n", svs.time );
 }
 
 /*
@@ -1101,6 +1113,7 @@ qboolean SVD_PlaybackFrame( void ) {
 		return qfalse;
 	}
 
+
 	// manual pause — don't consume demo data
 	if ( demo.paused ) {
 		return qfalse;