Rewrite ARCHITECTURE.md and TODOS.md to match current codebase

[CREVIOS]

Problem

Both docs are severely outdated:

ARCHITECTURE.md

• Still says "Phase 1 — Stub"
• Describes frame delivery as "PNG compression" and "base64_data"
• Doesn't mention: SharedFrame, GpuRenderer, wgpu, H.264 encoder, condvar signaling, dedicated render thread, or the transparent webview overlay plan
• The session state machine diagram is correct but incomplete (missing reconnect cancellation)

TODOS.md

• Lists "P1 — Enable Full IronRDP Protocol Negotiation" as remaining — but this is DONE (connected to live Windows servers)
• Doesn't mention any of the GPU rendering work
• Doesn't reference the 13 GitHub issues
• The completed items are accurate but the remaining items are wrong

What needs updating

ARCHITECTURE.md should describe:

1. The unified GPU pipeline: IronRDP → SharedFrame → GpuRenderer → Metal/Vulkan
2. The transparent webview overlay pattern (Layer 0: wgpu, Layer 1: WebView)
3. The dual-path architecture (GPU primary, IPC Canvas fallback)
4. The session actor with SharedFrame integration
5. The render thread model (dedicated OS thread, condvar-based)
6. The H.264 encoder (for future session recording)

TODOS.md should:

1. Move IronRDP negotiation to Completed
2. Add completed items: GPU renderer, SharedFrame, H.264 encoder, scroll support, keyboard fix
3. Update remaining items to match GitHub issues Add connection card thumbnail (save screenshot on disconnect) #1-13
4. Add new P0: transparent webview overlay (issue Make wgpu GPU-rendered frames visible via transparent webview overlay #14)

Files to modify

• `ARCHITECTURE.md`
• `TODOS.md`

Priority: P1 — stale docs actively mislead contributors