replace webview with 2D canvas builtins (winit + tiny-skia + fontdue)

Removes wry/tao WebView layer. Adds a pixel-level canvas API so Engram
programs can drive the window directly — fill rects, draw text, handle
mouse/keyboard input — enabling the UI framework to be written in Engram.

Dependencies: winit 0.29 (rwh_05), softbuffer 0.3, tiny-skia 0.11, fontdue 0.8.

Cargo.toml

@@ -58,3 +58,7 @@ hmac        = "0.12"
 sha2        = "0.10"
 hex         = "0.4"
 base64      = "0.22"
+winit       = { version = "0.29", default-features = false, features = ["rwh_05"] }
+softbuffer  = "0.3"
+tiny-skia   = "0.11"
+fontdue     = "0.8"