feat: add paintKit field to Sticker for sticker slab identification

Proto field 12 inside the Sticker sub-message encodes the actual sticker
slab variant ID. Previously this field was silently ignored, causing all
Sticker Slabs to decode identically (stickerId=37 for all variants).

paintKit maps to the sticker catalog ID (stickers.json) and uniquely
identifies which Sticker Slab variant an item is.

Add decode/encode support and two test vectors:
- NaVi (Holo) | Copenhagen 2024: defIndex=1355, rarity=5, paintKit=7256
- Eco Rush: defIndex=1355, rarity=3, paintKit=275

Author: vlydev

src/InspectLink.php

@@ -220,6 +220,10 @@ private static function encodeSticker(Sticker $s): string
             $w->writeUint32(11, $s->highlightReel);
         }
 
+        if ($s->paintKit !== null) {
+            $w->writeUint32(12, $s->paintKit);
+        }
+
         return $w->toBytes();
     }
 
@@ -241,6 +245,7 @@ private static function decodeSticker(string $data): Sticker
                 9 => $s->offsetZ = (float) unpack('f', $f['value'])[1],
                 10 => $s->pattern = $f['value'],
                 11 => $s->highlightReel = $f['value'],
+                12 => $s->paintKit = $f['value'],
                 default => null,
             };
         }

src/Sticker.php

@@ -24,5 +24,6 @@ public function __construct(
         public ?float $offsetZ = null,
         public int $pattern = 0,
         public ?int $highlightReel = null,
+        public ?int $paintKit = null,
     ) {}
 }

tests/InspectLinkTest.php

@@ -570,4 +570,86 @@ public function testSerialize_NullPaintwearProducesFewerBytes(): void
         $withFloat = InspectLink::serialize(new ItemPreviewData(defindex: 7, paintwear: 0.5));
         $this->assertLessThan(strlen($withFloat), strlen($withNull));
     }
+
+    // -----------------------------------------------------------------------
+    // Sticker Slab test vectors (defIndex=1355, quality=8, sticker slab variant via paintKit)
+    // -----------------------------------------------------------------------
+
+    /** Sticker Slab URL A — rarity=5, keychains[0].stickerId=37, keychains[0].paintKit=7256 */
+    private const SLAB_URL_A = 'steam://run/730//+csgo_econ_action_preview%20918191895A9BB191B994A199F991E191339096999181B4F149A98D5C0889';
+
+    /** Sticker Slab URL B — rarity=3, keychains[0].stickerId=37, keychains[0].paintKit=275 */
+    private const SLAB_URL_B = 'steam://run/730//+csgo_econ_action_preview%20CBDBCBD300C1EBCBE3C8FBC3A3CBBBCB69CACCC3CBDBEEAB58C9B8B67C83';
+
+    public function testSlabA_Defindex(): void
+    {
+        $this->assertSame(1355, InspectLink::deserialize(self::SLAB_URL_A)->defindex);
+    }
+
+    public function testSlabA_Quality(): void
+    {
+        $this->assertSame(8, InspectLink::deserialize(self::SLAB_URL_A)->quality);
+    }
+
+    public function testSlabA_Rarity(): void
+    {
+        $this->assertSame(5, InspectLink::deserialize(self::SLAB_URL_A)->rarity);
+    }
+
+    public function testSlabA_KeychainStickerId(): void
+    {
+        $keychains = InspectLink::deserialize(self::SLAB_URL_A)->keychains;
+        $this->assertCount(1, $keychains);
+        $this->assertSame(37, $keychains[0]->stickerId);
+    }
+
+    public function testSlabA_KeychainPaintKit(): void
+    {
+        $keychains = InspectLink::deserialize(self::SLAB_URL_A)->keychains;
+        $this->assertCount(1, $keychains);
+        $this->assertSame(7256, $keychains[0]->paintKit);
+    }
+
+    public function testSlabB_Defindex(): void
+    {
+        $this->assertSame(1355, InspectLink::deserialize(self::SLAB_URL_B)->defindex);
+    }
+
+    public function testSlabB_Quality(): void
+    {
+        $this->assertSame(8, InspectLink::deserialize(self::SLAB_URL_B)->quality);
+    }
+
+    public function testSlabB_Rarity(): void
+    {
+        $this->assertSame(3, InspectLink::deserialize(self::SLAB_URL_B)->rarity);
+    }
+
+    public function testSlabB_KeychainStickerId(): void
+    {
+        $keychains = InspectLink::deserialize(self::SLAB_URL_B)->keychains;
+        $this->assertCount(1, $keychains);
+        $this->assertSame(37, $keychains[0]->stickerId);
+    }
+
+    public function testSlabB_KeychainPaintKit(): void
+    {
+        $keychains = InspectLink::deserialize(self::SLAB_URL_B)->keychains;
+        $this->assertCount(1, $keychains);
+        $this->assertSame(275, $keychains[0]->paintKit);
+    }
+
+    public function testRoundtrip_PaintKit(): void
+    {
+        $data = new ItemPreviewData(
+            defindex: 1355,
+            quality: 8,
+            rarity: 5,
+            keychains: [new Sticker(slot: 0, stickerId: 37, paintKit: 7256)],
+        );
+        $result = InspectLink::deserialize(InspectLink::serialize($data));
+        $this->assertCount(1, $result->keychains);
+        $this->assertSame(37, $result->keychains[0]->stickerId);
+        $this->assertSame(7256, $result->keychains[0]->paintKit);
+    }
 }