Track remaining engine/runtime gaps for full Bevy 2D example parity

[Milky2018]

Context

This issue tracks lower-level gaps found while aligning all examples under mgstudio-engine/examples/2d with bevy/examples/2d.

Runtime / API gaps

• Missing per-load image sampler settings equivalent to Bevy AssetServer::load_with_settings and ImagePlugin::default_nearest().
• Missing WindowResized-style event reader API for strict event-driven resize behavior.
• Missing Bevy-like 2D wireframe stack:
  • Wireframe2dPlugin
  • Wireframe2dConfig
  • Wireframe2d
  • NoWireframe2d
  • Wireframe2dColor
  • native POLYGON_MODE_LINE pipeline mode exposure.
• Missing custom 2D material/pipeline parity for dynamic mip sample control (Material2dPlugin / explicit mip-level sampling style).
• Missing unified single-texture multi-mip runtime surface with Bevy-like GPU sampling semantics (current model uses split handles for mip chain workflows).
• Missing circular mesh UV builder parity (CircularMeshUvMode / CircularSectorMeshBuilder-equivalent controls).
• Missing SpriteImageMode::Scale(SpriteScalingMode::...) parity.
• Missing richer text styling parity for examples (TextBackgroundColor, underline/rich-span completeness).
• Missing image array-layout loader parity used by tilemap-style examples (ImageLoaderSettings::array_layout equivalent).

Asset/content gaps

• Add Bevy-equivalent assets used by examples where absent in mgstudio-engine/assets:
  • textures/rpg/chars/gabe/gabe-idle-run.png
  • textures/branding/banner.png
  • textures/slice_square.png
  • textures/slice_square_2.png

Determinism / utility parity

• Missing ChaCha8-style RNG parity helper (examples currently use deterministic fallback RNG).

Goal

Close these gaps to move current "best-effort parity" examples to strict 1:1 behavior parity with Bevy 2D examples.