simplify simulator around local participant core

[rksm]

Summary

Implements the 2026-03-27 simplification plan by collapsing the simulator around the local participant core.

What Changed

• removes the orchestrator, HTTP worker, and stats-gatherer crates from the active workspace and related Nix/Just wiring
• replaces the websocket-based remote participant path with a remote stub behind the existing participant interface
• makes backend selection explicit instead of inferring "remote" from a saved remote URL index
• extracts shared frontend runtime logic so hyper.video and hyper-lite reuse the same browser lifecycle flow
• updates config, defaults, and docs to match the simplified architecture
• fixes the Nix dev shell runtime library path so just run can resolve OpenSSL at runtime

Touched Areas

• root crate
• browser
• config
• tui
• removed http, orchestrator, and stats-gatherer
• Nix and Just configuration

Validation

• nix develop --command cargo check --workspace
• nix develop --command bash -lc 'target/release/client-simulator --help'