SVG tiling: fast tile generator, perf improvements, bug fixes

[gentledepp]

Core:

• Fast/efficient tile generator for large images, incl. direct SVG tiling
• Consolidated zip/png/svg loading into a single tool command
• PlaceAsBackgroundTool runs conversions on background thread

Performance:

• Default maxParallelTasks to Environment.ProcessorCount (was unlimited → SEHException risk)
• SKFilterQuality.Medium when downsampling tiles
• Visibility check before LoadTile in sync RenderBitmap (skip offscreen tiles)
• LoadTileStream/Async return FileStream directly, no MemoryStream copy
• Shared ZipArchive per render pass in SvgImage (was one per tile)
• LRU eviction in TileCache (was silently discarding new items when full)
• Configurable tile encoding (format/quality, default PNG 100)

Fixes:

• Editor crash on window resize (race between PropertyChange/Dispose and render thread)
• Incorrect viewbox calculation
• Zoom from tile + zoom limitation
• SvgImage not editable when added as tile
• TileCache.CleanUp bug
• Concurrent rendering issue

Misc:

• Removed obsolete base64 tile generation code
• Uncommented and fixed TileGenerator/TileRenderer tests
• MainWindow standard size