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_pipeline.svg

@@ -0,0 +1,51 @@
+<svg xmlns="http://www.w3.org/2000/svg" width="1200" height="640" viewBox="0 0 1200 640" role="img" aria-labelledby="title desc">
+  <title id="title">Background rendering: prepare a style once, reuse it while moving the camera</title>
+  <desc id="desc">A visible surface requests its finish and tint from the scene cache. A cache hit goes directly to the GPU repeat sampler. A cache miss uses one temporary baker, captures an image and mipmaps, and releases vector commands.</desc>
+  <defs>
+    <marker id="arrow" markerWidth="10" markerHeight="10" refX="8" refY="5" orient="auto"><path d="M0 0L10 5L0 10Z" fill="#475569"/></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:#475569;stroke-width:3;marker-end:url(#arrow)}</style>
+  </defs>
+  <rect width="1200" height="640" rx="16" fill="#f8fafc"/>
+  <text x="40" y="53" class="title">Prepare a material once. Reuse it as the camera moves.</text>
+  <text x="40" y="86" class="body">Only visible, committed patterned scenery requests a cached material.</text>
+
+  <rect x="40" y="140" width="250" height="126" rx="12" fill="#eff6ff" stroke="#2563eb" stroke-width="2"/>
+  <text x="60" y="177" class="heading">Visible surface</text>
+  <text x="60" y="210" class="body">Finish + tint</text>
+  <text x="60" y="239" class="small">Example: blue brick</text>
+  <path d="M292 203H370" class="arrow"/>
+
+  <rect x="390" y="140" width="260" height="126" rx="12" fill="#eff6ff" stroke="#2563eb" stroke-width="2"/>
+  <text x="410" y="177" class="heading">Scene style cache</text>
+  <text x="410" y="210" class="body">Already have this image?</text>
+  <text x="410" y="239" class="small">Shared by all matching surfaces</text>
+  <path d="M652 203H740" class="arrow"/>
+  <text x="674" y="182" class="small">Hit</text>
+
+  <rect x="760" y="140" width="400" height="126" rx="12" fill="#ecfdf5" stroke="#0f766e" stroke-width="2"/>
+  <text x="782" y="177" class="heading">GPU draws the visible rectangle</text>
+  <text x="782" y="210" class="body">Repeat texture + sample its mipmaps</text>
+  <text x="782" y="239" class="small">Pan / zoom updates sampling coordinates</text>
+
+  <path d="M520 268V324" class="arrow"/>
+  <text x="540" y="304" class="small">Miss: prepare this style</text>
+  <rect x="390" y="344" width="260" height="158" rx="12" fill="#f5f3ff" stroke="#7c3aed" stroke-width="2"/>
+  <text x="410" y="379" class="heading">One temporary baker</text>
+  <text x="410" y="412" class="body">Draw one pattern period</text>
+  <text x="410" y="441" class="body">Wait for GPU; capture image</text>
+  <text x="410" y="476" class="small">Then clear vector commands</text>
+  <path d="M652 423H740" class="arrow"/>
+
+  <rect x="760" y="344" width="400" height="158" rx="12" fill="#f5f3ff" stroke="#7c3aed" stroke-width="2"/>
+  <text x="782" y="379" class="heading">Store the reusable texture</text>
+  <text x="782" y="412" class="body">Crop overscan; generate mipmaps</text>
+  <text x="782" y="441" class="body">Keep the image, not the vector artwork</text>
+  <text x="782" y="476" class="small">Ready styles stay warm within the cache budget</text>
+  <path d="M960 342V282" class="arrow"/>
+  <text x="980" y="309" class="small">Ready</text>
+
+  <rect x="40" y="548" width="540" height="56" rx="10" fill="#dbeafe"/>
+  <text x="59" y="582" class="body">Blue route: reuse during ordinary pan and zoom</text>
+  <rect x="604" y="548" width="556" height="56" rx="10" fill="#ede9fe"/>
+  <text x="623" y="582" class="body">Purple route: bake only when a style is missing</text>
+</svg>