docs: propose Cua Driver environment convergence RFC

[f-trycua]

Landing scope

This PR records a proposal with status: review; merging it does not accept the broader architecture or authorize its migrations. The separately accepted Python/Fleet first slice is recorded in #3647 and the scoped maintainer decision. No runtime, release, or image change is included here.

Summary

• define one typed CuaDriver contract across hosts, local Cua Sandboxes, Lume macOS VMs, and Cua Fleet claims
• separate desktop control from environment lifecycle and services
• add Cua-Bench as the evaluation consumer of the same Driver and environment boundaries
• specify generated remote connection framing, lifecycle, authentication, platform requirements, telemetry, and normalized errors
• define a staged computer-server compatibility and migration path

Review focus

• whether environment SDKs should return the generated CuaDriver object
• whether the proposed generated remote connection preserves RFC 2447's dependency direction
• the claim/environment-scoped credential and stale-handle model
• the boundary between Cua Driver, Sandbox, Lume, Fleet, Cua-Bench, and computer-server
• whether the rollout and cross-platform acceptance gates are sufficient

Tracking

• Discussion: [RFC]: Converge Cua Driver across Sandbox, Lume, Fleet, and Cua-Bench #2512

Validation

• Prettier formatting check
• Markdown whitespace check
• local-link resolution
• public-disclosure and credential scan
• two Mermaid architecture diagrams included for review