controller: implement elevation mask

There's not really any analogous concept for azimuth. This is for a
specific piece of hardware, so there's no real point in making it more
generic. This is respected at the 180 degree point as well, for
software that can handle flipped rotation configurations.

This doesn't currently play well with the elevation offset setting,
which should be applied after this clamping operation. Either this
needs to be moved to the API layer or (more appropriately) the input
range validation needs to move in the controller.

src/Config.zig

@@ -65,6 +65,9 @@ controller: ControllerConfig = .{
     .parking_posture = .{ .azimuth = 180, .elevation = 90 },
     .angle_tolerance = .{ .azimuth = 1, .elevation = 1 },
     .angle_offset = .{ .azimuth = 0, .elevation = 0 },
+    // this is a symmetric mask, so the minimum usable elevation is elevation_mask deg
+    // and the maximum usable elevation is 180 - elevation_mask deg
+    .elevation_mask = 0.0,
 },
 
 pub const VoltAngle = struct { voltage: f64, angle: f64 };
@@ -115,6 +118,7 @@ const ControllerConfig = struct {
     parking_posture: AzEl,
     angle_tolerance: AzEl,
     angle_offset: AzEl,
+    elevation_mask: f64,
 
     const OutPair = struct {
         increase: lj.DigitalOutputChannel,

src/LabjackYaesu.zig

@@ -40,7 +40,13 @@ pub fn setTarget(self: LabjackYaesu, target: AzEl) void {
     defer self.lock.unlock();
 
     const controller = @constCast(self.controller);
-    controller.target = target;
+    controller.target = .{
+        .azimuth = target.azimuth,
+        .elevation = @min(
+            @max(target.elevation, config.controller.elevation_mask),
+            180.0 - config.controller.elevation_mask,
+        ),
+    };
     controller.requested_state = .running;
 }