pid: add pipeline on error signal computation

fast-servo/pid_err_sig_pipeline.patch

@@ -0,0 +1,19 @@
+diff --git a/gateware/logic/pid.py b/gateware/logic/pid.py
+index e737577..f1a4096 100644
+--- a/gateware/logic/pid.py
++++ b/gateware/logic/pid.py
+@@ -43,10 +43,12 @@ class PID(Module, AutoCSR):
+         self.comb += [setpoint_signed.eq(self.setpoint.storage)]
+ 
+         self.error = Signal((self.width + 1, True))
++        error_reg = Signal((self.width + 1, True))
++        self.sync += self.error.eq(error_reg)
+ 
+         self.comb += [
+-            If(self.running, self.error.eq(self.input - self.setpoint.storage)).Else(
+-                self.error.eq(0)
++            If(self.running, error_reg.eq(self.input - self.setpoint.storage)).Else(
++                error_reg.eq(0)
+             )
+         ]
+ 

flake.nix

@@ -169,6 +169,7 @@
           fast-servo/iir_pipeline.patch
           fast-servo/linien_module_pipeline.patch
           fast-servo/pid_pipeline.patch
+          fast-servo/pid_err_sig_pipeline.patch
         ];
         nativeBuildInputs = [
           (pkgs.python3.withPackages(ps: [