Add tests for scenery surface caching and preview behavior

- Introduced `test_scenery_brick_preview.gd` to validate focused placement and CPU draw-cost for large brick walls.
- Added `test_scenery_surface_cache.gd` to assess texture budget, visual equivalence, and cache invalidation.
- Created `test_scenery_surface_preview.gd` to evaluate adaptive preview selection and placement behavior.
- Implemented various helper functions to measure drawing performance and validate expected behaviors during tests.
- Added unique identifiers for new test files to ensure proper tracking and management.

docs/architecture/diagrams/background_reuse.svg

@@ -0,0 +1,44 @@
+<svg xmlns="http://www.w3.org/2000/svg" width="1200" height="650" viewBox="0 0 1200 650" role="img" aria-labelledby="title desc">
+  <title id="title">Many wall rectangles share one material image</title>
+  <desc id="desc">One blue-brick image supplies Wall A, Wall B, and an erased fragment. Different sizes, positions, and camera views do not create more images. A different tint or finish is a different cached style.</desc>
+  <defs>
+    <pattern id="brick" width="64" height="40" patternUnits="userSpaceOnUse">
+      <rect width="64" height="40" fill="#d2cbaf"/>
+      <g><rect x="2" y="2" width="28" height="15" fill="#25b8df"/><rect x="34" y="2" width="28" height="15" fill="#39c7ee"/><rect x="-14" y="22" width="28" height="15" fill="#32c2e8"/><rect x="18" y="22" width="28" height="15" fill="#21b4da"/><rect x="50" y="22" width="28" height="15" fill="#32c2e8"/></g>
+    </pattern>
+    <marker id="arrow" markerWidth="10" markerHeight="10" refX="8" refY="5" orient="auto"><path d="M0 0L10 5L0 10Z" fill="#2563eb"/></marker>
+    <style>text{font-family:Segoe UI,Arial,sans-serif;fill:#172033}.title{font-size:30px;font-weight:700}.heading{font-size:22px;font-weight:700}.body{font-size:18px}.small{font-size:16px}.arrow{fill:none;stroke:#2563eb;stroke-width:3;marker-end:url(#arrow)}</style>
+  </defs>
+  <rect width="1200" height="650" rx="16" fill="#f8fafc"/>
+  <text x="40" y="53" class="title">Wall area can grow. The number of style images stays the same.</text>
+  <text x="40" y="86" class="body">Position, wall size, camera position and zoom are not part of the style key.</text>
+
+  <rect x="400" y="117" width="400" height="163" rx="12" fill="#eff6ff" stroke="#2563eb" stroke-width="2"/>
+  <rect x="420" y="140" width="160" height="112" fill="url(#brick)" stroke="#94a3b8"/>
+  <text x="602" y="162" class="heading">One shared image</text>
+  <text x="602" y="194" class="body">Blue brick</text>
+  <text x="602" y="224" class="small">Varied brick faces</text>
+  <text x="602" y="250" class="small">Repeats both ways</text>
+  <path d="M600 282V311H210V341" class="arrow"/>
+  <path d="M600 311V341" class="arrow"/>
+  <path d="M600 311H990V341" class="arrow"/>
+
+  <rect x="40" y="360" width="340" height="192" rx="12" fill="#fff" stroke="#cbd5e1" stroke-width="2"/>
+  <text x="62" y="396" class="heading">Wall A</text>
+  <rect x="62" y="416" width="296" height="103" fill="url(#brick)"/>
+
+  <rect x="430" y="360" width="340" height="192" rx="12" fill="#fff" stroke="#cbd5e1" stroke-width="2"/>
+  <text x="452" y="396" class="heading">Wall B</text>
+  <rect x="452" y="416" width="190" height="103" fill="url(#brick)"/>
+  <text x="659" y="455" class="small">Same</text>
+  <text x="659" y="480" class="small">style</text>
+
+  <rect x="820" y="360" width="340" height="192" rx="12" fill="#fff" stroke="#cbd5e1" stroke-width="2"/>
+  <text x="842" y="396" class="heading">Erased fragment</text>
+  <rect x="842" y="416" width="90" height="103" fill="url(#brick)"/>
+  <text x="952" y="455" class="small">Only the visible</text>
+  <text x="952" y="480" class="small">rectangle changes</text>
+
+  <text x="40" y="591" class="body">A different finish or tint is a separate style. A matching warm style reuses the existing image.</text>
+  <text x="40" y="621" class="small" fill="#64748b">Patterns shown here are schematic; the actual brick image contains 8 columns × 8 rows of varied artwork.</text>
+</svg>