Add split-repo compatibility gates for terraphim-ai server and desktop

[AlexMikhalev]

Problem

The split between terraphim-ai-desktop and the sibling terraphim-ai repo needs ongoing compatibility coverage. Today the manual compile/build checks pass, but the relationship is not yet enforced by a dedicated compatibility gate.

Goal

Add explicit desktop/server compatibility checks for the split-repo architecture.

Scope

• codify the validated contract surface between desktop web mode and terraphim_server
• add scripts or CI steps that run cargo check -p terraphim_server, desktop Rust checks, frontend build, and focused parity tests
• document expected server endpoints and Tauri command names used by the desktop app

Acceptance

• compatibility steps are scripted and documented
• regressions in server endpoints or desktop callers fail early
• desktop/server parity is testable without re-discovering contracts manually