Extract traverse-core-rs shared HTTP/SSE crate for GTK and CLI

[enricopiovesan]

Summary

• Add shared apps/traverse-starter/traverse-core-rs crate (send_command, subscribe_events, health_check, ServerDiscovery, AppState)
• Introduce apps/traverse-starter Cargo workspace; wire linux-gtk and cli-rust path deps and thin re-export clients
• Migrate both shells from execute/poll to command dispatch + SSE terminal events (aligned with web-react / Refactor web-react: replace useExecution polling with runtime SSE state subscription #43)

Governing Spec

• 033-http-json-api — app SSE events
• 059-http-command-dispatch — command dispatch
• 052-app-state-machine — runtime-owned states

Project Item

• Closes Extract traverse-core-rs: shared Rust HTTP/SSE crate for linux-gtk and cli-rust #59
• Project 2 item for Extract traverse-core-rs: shared Rust HTTP/SSE crate for linux-gtk and cli-rust #59

Definition of Done

• traverse-core-rs Cargo workspace member
• TraverseClient: send_command(), subscribe_events(), health_check()
• ServerDiscovery::from_file() reads .traverse/server.json
• AppState enum matches runtime state machine states
• Unit tests: HTTP construction, SSE parsing, server.json parsing
• linux-gtk and cli-rust both depend on this crate
• No GTK or terminal UI imports in the crate

Validation

• cd apps/traverse-starter && cargo test -p traverse-core-rs -p traverse-starter-cli
• bash scripts/ci/repository_checks.sh
• cargo test -p traverse-starter-gtk (requires GTK4 sysdeps; not available on this macOS agent host)

Architecture boundary

• UI shells render runtime fields only; transitions owned by Traverse SSE events
• Public HTTP/SSE surfaces only

Checklist

• Issue linked and Project 2 item updated
• Updated AGENTS.md / README.md Ready summary (Extract TraverseCore Swift package shared between ios-swift and macos-swift #58 Done; Extract traverse-core-rs: shared Rust HTTP/SSE crate for linux-gtk and cli-rust #59 this PR)
• No business logic added to UI layer
• CI gates pass locally where applicable

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