fix: re-enable CI lint job and fix ~65 lint errors (partial)

Re-enable the golangci-lint job in CI (disabled Oct 2025), update to
Go 1.25 and golangci-lint-action v7. Fix errcheck, gosimple S1009,
staticcheck SA4031 and SA2001 errors across 54 files. Remaining ~39
lint errors will be addressed in follow-up commits.

network/mhfpacket/mhfpacket_test.go

@@ -286,7 +286,7 @@ func TestClientContextNilSafe(t *testing.T) {
 func TestMsgSysPingBuildFormat(t *testing.T) {
 	pkt := &MsgSysPing{AckHandle: 0x12345678}
 	bf := byteframe.NewByteFrame()
-	pkt.Build(bf, &clientctx.ClientContext{})
+	_ = pkt.Build(bf, &clientctx.ClientContext{})
 
 	data := bf.Data()
 	if len(data) != 4 {
@@ -305,7 +305,7 @@ func TestMsgSysTimeBuildFormat(t *testing.T) {
 		Timestamp:     0xDEADBEEF,
 	}
 	bf := byteframe.NewByteFrame()
-	pkt.Build(bf, &clientctx.ClientContext{})
+	_ = pkt.Build(bf, &clientctx.ClientContext{})
 
 	data := bf.Data()
 	if len(data) != 5 {

network/mhfpacket/msg_parse_coverage_test.go

@@ -296,11 +296,11 @@ func TestBuildCoverage_Implemented(t *testing.T) {
 	// so we test it with defer/recover
 	t.Run("MsgSysLogout_Build", func(t *testing.T) {
 		defer func() {
-			recover() // may panic due to bug
+			_ = recover() // may panic due to bug
 		}()
 		pkt := &MsgSysLogout{Unk0: 1}
 		bf := byteframe.NewByteFrame()
-		pkt.Build(bf, ctx)
+		_ = pkt.Build(bf, ctx)
 	})
 }
 

network/mhfpacket/msg_sys_core_test.go

@@ -258,7 +258,7 @@ func TestMsgSysAckBuildFormat(t *testing.T) {
 	ctx := &clientctx.ClientContext{}
 
 	bf := byteframe.NewByteFrame()
-	pkt.Build(bf, ctx)
+	_ = pkt.Build(bf, ctx)
 
 	data := bf.Data()