Reduce Seraphim Destroyer underwater speed 5 -> 4.5 (#6588)

The Seraphim destroyer is currently too powerful due to its micro
potential, reducing the underwater speed allows
counterplay potential without heavily affecting unmicro'd Seraphim destroyer performance.

Aeon and Cybran destroyers can catch Seraphim destroyers that
overcommitted, and all destroyers (particularly UEF) can flee from
submerged Seraphim destroyers and get out of torpedo range.

changelog/snippets/balance.6588.md

@@ -0,0 +1,4 @@
+- (#6558) The Seraphim destroyer is currently too powerful due to its micro potential, but nerfing its main stats like range or damage would make it weaker than it already is when not micro'd, which would be unfun at lower levels. Considering this, the underwater speed is reduced to allow counterplay potential without heavily affecting lower level gameplay: Aeon and Cybran destroyers can catch Seraphim destroyers that overcommitted, and all destroyers (particularly UEF) can flee from submerged Seraphim destroyers and get out of torpedo range.
+
+    - Seraphim Destroyer (Uashavoh):
+      - Underwater speed: 5 -> 4.5

engine/Core/Blueprints/UnitBlueprint.lua

@@ -1170,7 +1170,7 @@
 ---@field SkirtSizeZ number
 --- Stands upright regardless of terrain
 ---@field StandUpright boolean
---- used by XSB3202 when the vertical layer changes from top to sub
+--- used by XSB3202 (T2 sonar) and XSS0201 (Destroyer) when the vertical layer changes from top to sub
 ---@field SubSpeedMultiplier? number
 --- How much this unit slows down transports it is loaded in.  
 --- Defaults to 0.15 (Tech 1), 0.3 (Tech 2), 0.6 (Tech 3), and 1 (ACU/SACU/Experimentals).

units/XSS0201/XSS0201_script.lua

@@ -55,14 +55,17 @@ XSS0201 = ClassUnit(SSubUnit) {
         if new == 'Top' then
             self:SetWeaponEnabledByLabel('FrontTurret', true)
             self:SetWeaponEnabledByLabel('BackTurret', true)
+            self:SetSpeedMult(1)
         elseif new == 'Down' then
             self:SetWeaponEnabledByLabel('FrontTurret', false)
             self:SetWeaponEnabledByLabel('BackTurret', false)
+            self:SetSpeedMult(self.Blueprint.Physics.SubSpeedMultiplier or 1)
         end
     end,
 
     OnStopBeingBuilt = function(self, builer, layer)
         SSubUnit.OnStopBeingBuilt(self, builer, layer)
+        self:SetSpeedMult(self.Blueprint.Physics.SubSpeedMultiplier or 1)
         if self.originalBuilder then
             IssueDive({self})
         end

units/XSS0201/XSS0201_unit.bp

@@ -164,6 +164,7 @@ UnitBlueprint{
         MeshExtentsY = 2,
         MeshExtentsZ = 6.25,
         MotionType = "RULEUMT_SurfacingSub",
+        SubSpeedMultiplier = 0.9,
         TurnRadius = 15,
         TurnRate = 50,
     },