fix: replace w3cos-build with workspace demo crate for reliable Docker builds

The `w3cos build` command creates temp cargo projects which fail in Docker
(Exec format error, path resolution issues). Instead, pre-generate the
showcase Rust code as a proper workspace member `w3cos-demo` that compiles
directly with `cargo build -p w3cos-demo`.

- Add crates/w3cos-demo/ with pre-generated showcase main.rs
- Simplify HF Space Dockerfile to use cargo build directly
- Update start.sh to use w3cos-demo binary

Made-with: Cursor

Author: wangnaihe

Cargo.toml

@@ -8,6 +8,7 @@ members = [
     "crates/w3cos-dom",
     "crates/w3cos-a11y",
     "crates/w3cos-ai-bridge",
+    "crates/w3cos-demo",
 ]
 
 [workspace.package]

crates/w3cos-demo/Cargo.toml

@@ -0,0 +1,14 @@
+[package]
+name = "w3cos-demo"
+version.workspace = true
+edition.workspace = true
+license.workspace = true
+description = "W3C OS showcase demo binary — pre-compiled from examples/showcase/app.tsx"
+
+[[bin]]
+name = "w3cos-demo"
+path = "src/main.rs"
+
+[dependencies]
+w3cos-runtime = { workspace = true }
+w3cos-std = { workspace = true }