Feature: Add Spall Liner toggle to Tankopedia model viewer

## Summary
- Add a Spall Liner toggle button to the Tankopedia model viewer, placed alongside the existing Calibrated Shells / Enhanced Armor buttons. It's only shown for tanks that can actually equip Spall Liner.
- The toggle reads/writes the same `Duel.protagonist.provisions` array as the Provisions tab, so both stay in sync automatically.
- If a provision slot is free, clicking equips Spall Liner immediately. If all slots are full, a popover shows icons with a shuffle arrow so you can pick which active provision to swap out in one step.
- Buttons use the same medium rounding as the consumable buttons on the tank page.
- When Spall Liner is equipped, the Dynamic Armor 3D viewer's click-damage tooltip now applies the same 20% HE damage reduction Spall Liner grants in-game.
- Refactored the hardcoded `-0.2` HE damage coefficient in `tankCharacteristics.ts` into a shared constant so the Compare tool and the 3D viewer use a single source of truth.

## Scope
This only affects the single-tank Tankopedia page. The Firepower stats table (the viewed tank's own outgoing gun damage) is intentionally untouched, since a defensive damage reduction would be semantically backwards there. The Dynamic Armor viewer's click tooltip is the only place a "damage taken by this tank" number exists, so that's where the reduction is wired in.

packages/website/src/components/Tankopedia/HeroSection/components/Options/components/SpallLinerSwitcher.tsx

@@ -0,0 +1,128 @@
+import { alias, availableProvisions } from "@blitzkit/core";
+import { ShuffleIcon } from "@radix-ui/react-icons";
+import { Flex, IconButton, Popover } from "@radix-ui/themes";
+import { api } from "../../../../../../core/blitzkit/api";
+import { SPALL_LINER_PROVISION_ID } from "../../../../../../core/blitzkit/spallLiner";
+import { useLocale } from "../../../../../../hooks/useLocale";
+import { Duel } from "../../../../../../stores/duel";
+import { Tankopedia } from "../../../../../../stores/tankopedia";
+
+const provisionDefinitions = await api.provisionDefinitions();
+
+export function SpallLinerSwitcher() {
+  const { tank, gun, provisions } = Duel.use((state) => state.protagonist);
+  const { unwrap } = useLocale();
+  const isSpallLinerActive = provisions.includes(SPALL_LINER_PROVISION_ID);
+  const hasSlotAvailable = provisions.length < tank.max_provisions;
+
+  const canEquipSpallLiner = availableProvisions(
+    tank,
+    gun,
+    provisionDefinitions,
+  ).some((provision) => provision.id === SPALL_LINER_PROVISION_ID);
+
+  if (!canEquipSpallLiner) return null;
+
+  function equip(replacing?: number) {
+    Duel.mutate((draft) => {
+      if (replacing !== undefined) {
+        const provisions = draft.protagonist.provisions;
+        provisions[provisions.indexOf(replacing)] = SPALL_LINER_PROVISION_ID;
+      } else {
+        draft.protagonist.provisions.push(SPALL_LINER_PROVISION_ID);
+      }
+    });
+    Tankopedia.mutate((draft) => {
+      draft.shot = undefined;
+    });
+  }
+
+  function unequip() {
+    Duel.mutate((draft) => {
+      draft.protagonist.provisions = draft.protagonist.provisions.filter(
+        (id) => id !== SPALL_LINER_PROVISION_ID,
+      );
+    });
+    Tankopedia.mutate((draft) => {
+      draft.shot = undefined;
+    });
+  }
+
+  const button = (
+    <IconButton
+      color={isSpallLinerActive ? undefined : "gray"}
+      variant="soft"
+      size={{ initial: "2", sm: "3" }}
+      radius="medium"
+      onClick={() => {
+        if (isSpallLinerActive) {
+          unequip();
+        } else if (hasSlotAvailable) {
+          equip();
+        }
+      }}
+    >
+      <img
+        alt={unwrap(provisionDefinitions.provisions[SPALL_LINER_PROVISION_ID].name!)}
+        src={alias("api", `/icons/provisions/${SPALL_LINER_PROVISION_ID}.webp`)}
+        style={{
+          width: "50%",
+          height: "50%",
+        }}
+      />
+    </IconButton>
+  );
+
+  if (isSpallLinerActive || hasSlotAvailable) return button;
+
+  return (
+    <Popover.Root>
+      <Popover.Trigger>{button}</Popover.Trigger>
+
+      <Popover.Content>
+        <Flex gap="2" align="center">
+          <IconButton color="gray" variant="soft" size="3" radius="medium" disabled>
+            <img
+              alt={unwrap(
+                provisionDefinitions.provisions[SPALL_LINER_PROVISION_ID].name!,
+              )}
+              src={alias(
+                "api",
+                `/icons/provisions/${SPALL_LINER_PROVISION_ID}.webp`,
+              )}
+              style={{
+                width: "50%",
+                height: "50%",
+              }}
+            />
+          </IconButton>
+
+          <ShuffleIcon color="var(--gray-9)" />
+
+          <Flex gap="2">
+            {provisions.map((id) => (
+              <Popover.Close key={id}>
+                <IconButton
+                  color="gray"
+                  variant="soft"
+                  size="3"
+                  radius="medium"
+                  onClick={() => equip(id)}
+                >
+                  <img
+                    alt={unwrap(provisionDefinitions.provisions[id].name!)}
+                    src={alias("api", `/icons/provisions/${id}.webp`)}
+                    style={{
+                      width: "50%",
+                      height: "50%",
+                    }}
+                  />
+                </IconButton>
+              </Popover.Close>
+            ))}
+          </Flex>
+        </Flex>
+      </Popover.Content>
+    </Popover.Root>
+  );
+}

packages/website/src/components/Tankopedia/HeroSection/components/Options/index.tsx

@@ -46,6 +46,7 @@ import { TankSearch } from "../../../../TankSearch";
 import { CustomShellButton } from "./components/CustomShellButton";
 import { DynamicArmorSwitcher } from "./components/DynamicArmorSwitcher";
 import { QuickInputs } from "./components/QuickInputs";
+import { SpallLinerSwitcher } from "./components/SpallLinerSwitcher";
 import { Thicknesses } from "./components/Thicknesses";
 
 type OptionsProps = MaybeSkeletonComponentProps & {
@@ -278,6 +279,8 @@ export function Options({ thicknessRange, canvas, skeleton }: OptionsProps) {
               }}
             />
           </IconButton>
+
+          <SpallLinerSwitcher />
         </Flex>
 
         {!skeleton && (