Fixed

• External content preservation with MSAA (#455, #457 by @besmpl) — when gg renders an overlay on top of external 3D content (e.g. g3d scene), MSAA vector/mixed frames now use a transparent resolve + alpha-composite path that preserves the underlying content without duplicating pipeline variants or reducing antialiasing quality. Ordinary frames have zero overhead.
• External content overwritten by compositor base (#452, #453) — DrawGPUTextureBase no longer covers preserved g3d output with an opaque textured quad when PreserveContent is active.
• FlushGPUWithView ignored external content (#449, #450) — added PreserveContent flag to GPURenderTarget; render session uses LoadOp::Load instead of Clear when set.
• ggcanvas did not forward content preservation (#451) — Canvas.Render now detects ContentPreserver capability and forwards the state to the GPU render target.
• ggcanvas did not use shared command encoder (#454) — Canvas.Render now detects CommandEncoderProvider on the render target and records into the borrowed frame encoder for single-submit compositing with gogpu.

Changed

• Updated gogpu/gpucontext v0.22.0 → v0.23.0 (InputEvent sealed interface).
• Updated gogpu/wgpu v0.30.25 → v0.30.27 (GLES depth/stencil fix, MSAA external content preservation).