Windows: real CSS-style box-shadow via DirectComposition (#210 follow-up) #230

Context

#210 wired 4 of the 5 Windows styling stubs (decoration / opacity / border_color / border_width) at v0.5.347. The fifth — widget.shadow — remained Stub. Storage was already in place at crates/perry-ui-windows/src/widgets/mod.rs::SHADOW_PARAMS; what's missing is the rendering pass.

Why a separate issue

Real CSS box-shadow rendering on Win32 child controls (vs top-level windows) needs DirectComposition:

• IDCompositionDevice / IDCompositionVisual / IDCompositionEffectGroup
• IDCompositionDropShadowEffect (the named CSS-equivalent shape: color, blur, offset)
• DXGI swap chain integration so DirectComposition can composite atop the GDI render

Or alternatively a custom alpha-blended WM_PAINT pass on the parent that draws N concentric translucent rectangles to approximate Gaussian blur. This is the classic GDI "fake shadow" — visible but not pixel-accurate.

Either path is multi-day work that didn't fit inside #210's scope. Decoration / opacity / border could land via small per-prop apply functions; shadow needs new infrastructure.

Acceptance criteria

• widget.shadow row in the styling matrix flips from Stub → Wired for Windows.
• A widget calling widgetSetShadow(handle, r, g, b, a, blur, offsetX, offsetY) produces a visible drop shadow under the widget's bounds, matching the CSS box-shadow reference within reasonable approximation (color, offset, and at least 8-16px of blur radius).
• Existing widgets with corner radius (which uses SetWindowRgn) still render correctly — the shadow path must compose with the rounded clip, not fight it.
• docs/examples/ui/styling/visual_test.ts section 4 (Shadow) renders visible shadows on Windows.

Hints for the implementer

• crates/perry-ui-macos/src/widgets/mod.rs::set_shadow is the reference (CALayer-based — sets setMasksToBounds: false + shadowOpacity / shadowRadius / shadowOffset). Behavior to match: opaque shadow color with alpha riding on shadowOpacity; positive offsetY = downward (HTML convention).
• The SHADOW_PARAMS store already exists (mod.rs:798) with the full 7-tuple. The apply_corner_radius deferred-paint pattern at mod.rs:962 is the canonical "call from layout pipeline" hook — apply_shadow(handle) should slot in symmetrically.
• DirectComposition requires Windows 8+ (matches Perry's minimum). The windows-rs crate already enables Win32_Graphics_DirectComposition would need to be added to crates/perry-ui-windows/Cargo.toml's windows features.
• Two-stage strategy that keeps initial scope manageable: (1) GDI alpha-blend approximation first — visible shadow but not perfectly Gaussian; (2) DirectComposition upgrade later for true CSS parity. This issue could be split if the GDI step is enough for a v1.

Closes

When this lands: Windows reaches 43/43 Wired alongside every other Perry-supported platform. Combined with #210 (now closed except this row), the styling matrix is fully green for issue #185.