[sysid] Fix crash on negative feedforward gains (wpilibsuite#6425)

LinearSystemId's linear system factories throw on negative feedforward
gains, but SysId can compute the feedback gains just fine in that case.
Now we construct the system manually instead.

Fixes wpilibsuite#6423.

sysid/src/main/native/cpp/analysis/FeedbackAnalysis.cpp

@@ -8,7 +8,6 @@
 
 #include <frc/controller/LinearQuadraticRegulator.h>
 #include <frc/system/LinearSystem.h>
-#include <frc/system/plant/LinearSystemId.h>
 #include <units/acceleration.h>
 #include <units/velocity.h>
 #include <units/voltage.h>
@@ -32,8 +31,10 @@ FeedbackGains sysid::CalculatePositionFeedbackGains(
   // instabilities in the LQR.
   if (Ka > 1E-7) {
     // Create a position system from our feedforward gains.
-    auto system = frc::LinearSystemId::IdentifyPositionSystem<units::meter>(
-        Kv_t(Kv), Ka_t(Ka));
+    frc::LinearSystem<2, 1, 1> system{
+        frc::Matrixd<2, 2>{{0.0, 1.0}, {0.0, -Kv / Ka}},
+        frc::Matrixd<2, 1>{0.0, 1.0 / Ka}, frc::Matrixd<1, 2>{1.0, 0.0},
+        frc::Matrixd<1, 1>{0.0}};
     // Create an LQR with 2 states to control -- position and velocity.
     frc::LinearQuadraticRegulator<2, 1> controller{
         system, {params.qp, params.qv}, {params.r}, preset.period};
@@ -74,8 +75,9 @@ FeedbackGains sysid::CalculateVelocityFeedbackGains(
   }
 
   // Create a velocity system from our feedforward gains.
-  auto system = frc::LinearSystemId::IdentifyVelocitySystem<units::meter>(
-      Kv_t(Kv), Ka_t(Ka));
+  frc::LinearSystem<1, 1, 1> system{
+      frc::Matrixd<1, 1>{-Kv / Ka}, frc::Matrixd<1, 1>{1.0 / Ka},
+      frc::Matrixd<1, 1>{1.0}, frc::Matrixd<1, 1>{0.0}};
   // Create an LQR controller with 1 state -- velocity.
   frc::LinearQuadraticRegulator<1, 1> controller{
       system, {params.qv}, {params.r}, preset.period};