fix: prevent duplicate concurrent fetches and white images in RasterizingLayer - #3357

What changed

Several issues were found while investigating failures in the rendering regression tests.

RasterizingLayer (main fix)

• Prevent duplicate concurrent fetches: GetFetchJobs now returns an empty array when activeFetchCount > 0, matching the same guard already present in Layer.GetFetchJobs. Without this, DataFetcher.ViewportChangedAsync could start a second rasterize task while the first was still loading data, causing the layer to render a white image.
• Use the correct FetchJob ID: FetchJob was constructed with _sourceLayer.Id instead of the rasterizing layer's own Id. This made the activeFetchCount guard ineffective because job tracking is keyed by layer ID.

TwoStepSymbolStyleRenderer / TwoStepVectorStyleRenderer (experimental renderer)

• Both now implement ISkiaStyleRenderer as a fallback for cache misses. A ThemeStyle creates a new style instance on every GetStyle() call, producing a new GenerationId each time — so the drawable cache always misses, and features were silently not rendered. The fallback delegates directly to the underlying SymbolStyleRenderer / VectorStyleRenderer and fixes this case (e.g. SelectionStyleSample).

VisibleFeatureIterator

• layerStyles is now materialised to a list before iteration (minor cleanup, no behavioral change).

CustomPointStyleShaderSample

• The custom renderer was only registered on Mapsui.Rendering.Skia.MapRenderer. Added the matching registration on Mapsui.Experimental.Rendering.Skia.MapRenderer, consistent with CustomPointStyleBasicSample and CustomPointStyleAdvancedSample.

Regression test config

• Removed the test-project-level config.json / config.local.json lookup. TestConfiguration now reads only from the repository root, so there is a single authoritative config location.

Skill / documentation

• Updated regression-tests skill to clarify that the copy-images script is for intentional rendering changes only, and that the experimental renderer must produce pixel-identical output to the standard renderer for all non-ExperimentalOnlySamples samples.