Epic: GPUI renderer backend for rangular (native host)

[Interchouette]

Goal

Add a GPUI rendering backend to rangular so the same Angular-subset templates + Rust controllers can target:

• Web: Leptos → DOM (existing v0.1 path)
• Native: GPUI → GPU desktop UI (new)

Context

RustaShop wants Angular-shaped authoring across web and native GPU apps (see Interchouette-ITC/RustaShop docs-dev/UI-RENDERERS.md, RustaShop #50).

v0.1 SPEC explicitly excludes native widget toolkits; this epic is post-v0.1.

Outcomes

• Renderer trait / backend selection in toolchain
• GPUI spike: one rangular component (button + @if) on GPUI
• Document differences vs Leptos DOM (events, styling, layout)
• CI smoke for GPUI backend

Non-goals

• Replace Leptos web path
• Full Angular feature parity on day one

[Interchouette]

Child tasks: #38 (Renderer trait), #39 (GPUI spike), #40 (DOM vs GPUI doc), #41 (CI smoke).

Suggested order: 38 → 39 → 40 → 41.