Add per-axis joystick deadzone and sensitivity options

[dustinspace217]

What?

Adds Heading, Pitch, Bank and Relative Throttle deadzone and sensitivity options. Each defaults to "Use Global", which keeps using the existing Deadzone and Sensitivity settings; a value set for one of them applies to whatever stick axis is bound to that action. Addresses #4480.

Why?

Different axes on the same device often need different settings. The example in #4480 is a twist axis that needs a bigger deadzone than the main X and Y axes, which today means either living with a drifting twist or a mushy stick. MjnMixael asked that the new settings go through the options manager like the global ones, so that's how they're exposed.

How?

joy_get_scaled_reading() takes the axis action as an optional argument. When a per-action deadzone or sensitivity is set, it's used; otherwise the global value is, exactly as before. A per-action deadzone also takes precedence over -deadzone, since it's the more specific setting. Absolute throttle isn't included because it never used deadzone or sensitivity.

The settings are per action rather than per physical device and axis: a value applies to every joystick axis bound to that action (an action has a primary and a secondary binding). That covers the twist case in the issue, and it keeps the list to eight options with names players recognize, instead of thirty-two "Joystick 2, axis 5" entries. If per-device settings are still wanted, they could sit underneath these later.

• The eight per-axis rows sit under the global Sensitivity and Deadzone rows. A per-axis deadzone applies to its axis even when -deadzone is on the command line; the global slider is the one -deadzone replaces.

Testing?

• Linux, GCC, Release: builds with 0 warnings; unittests pass (new JoyScalingTest, 8 tests).
• JoyScalingTest covers: a per-axis deadzone overriding the global one and beating -deadzone; a per-axis sensitivity overriding the global one and falling back when unset; a per-axis deadzone using exactly the global deadzone's scale; actions outside the range using the global settings; a stick axis bound through scale_invert() picking up its action's deadzone; and each of the eight options binding to its own action and being an integer range. Removing the call-site change, changing the deadzone scale, binding one option to another action, or dropping the integer flag each fails a named test.
• In-game, the eight options appear in the Input section showing "Use Global", next to the unchanged global Deadzone and Sensitivity.
• Not tested with a physical joystick (the headless test setup has none).

Anything Else?

I used Claude Code to trace the joystick scaling path and to write the feature and tests.