Fix Default Stages: prevent nil newState causing offExtras to toggle incorrectly

[Copilot]

On every third light toggle, offExtras would turn ON instead of OFF when Default Stages feature was enabled.

Root Cause

In ULC:SetStage, the newState determination logic left newState as nil when an extra was already in its desired state:

-- Before: newState remains nil in these cases
if IsVehicleExtraTurnedOn(MyVehicle, extra) then
    if action == 1 or action == 2 then
        newState = 1
    end
    -- Missing: action == 0 leaves newState = nil
else
    if action == 0 or action == 2 then
        newState = 0
    end
    -- Missing: action == 1 leaves newState = nil
end

When setDefaultStages() calls with forceChange = true, processing continues despite nil newState, passing it to SetVehicleExtra() which interprets it incorrectly.

Changes

/ulc/client/c_buttons.lua - Added explicit newState assignments for both edge cases:

• When extra is ON and action is 0 (enable): set newState = 0
• When extra is OFF and action is 1 (disable): set newState = 1

This ensures newState is always defined when forceChange bypasses the early return check.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Bug: Unidentified bug with Default Stages</issue_title>
<issue_description>https://discord.com/channels/603591936372244501/1382451717743841421/1383197121498583040</issue_description>

<agent_instructions>make sure to review /docs for architecture info then attempt to identify and propose a fix for this bug.</agent_instructions>

Comments on the Issue (you are @copilot in this section)

@Flohhhhh It seems what's happening is every 1 in 3 Q presses (first being turn lights on, second turn lights off, third back on again) is instead of turning OFF "offExtras" it turns them ON instead.

It must be a bug related to the default stages feature since this only happens when light state toggles which is when default stages triggers.</comment_new>

• Fixes Bug: Unidentified bug with Default Stages #53

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