feat(startup): configurable scene load on boot and save-on-quit

[joschaschmiedt]

Summary

vstimd can now boot into a known scene with no client attached, controlled by a new rig-config [startup] section:

[startup]
# Named config in --config-dir to load at boot; "last" = the auto-saved
# last-session slot; omit / "" = empty scene.
load_config  = "center_target"
# Save the current scene on graceful shutdown, so load_config = "last" restores it.
save_on_quit = false

Behaviour

• Startup load (main.rs): after VTL init, an explicit --config <path> still wins; otherwise [startup] load_config applies. "last" (case-insensitive) resolves to a reserved slot (_last_session). A missing last-session slot on first boot is a logged no-op; a missing named config logs an error.
• Save on quit (main.rs): once the render loop returns (shutdown requested), the scene + VTL names are written to the last-session slot. Locks scene-then-vtl to match the ZMQ thread's order (ipc.rs), which is still live during shutdown — the reverse order would risk a deadlock mid-request.

Refactors (reuse / testability)

• io_config: new LAST_SESSION_CONFIG constant and config_path(dir, name) — the single place the vstimd_<name>.config.json layout lives (previously duplicated).
• SceneState::{load,save}_named_config — reusable methods shared by the LoadConfig command and the boot path.
• rig_config: StartupRigConfig + StartupLoad enum with a custom deserializer.

Tests

• server/tests/rig_config.rs: startup parsing (named, "last" incl. case-insensitivity, empty string, unknown-field rejection).
• server/tests/startup_config.rs (new): save→load roundtrip, expected file path, dir auto-creation, not-found reporting, last-session slot roundtrip, replace-clears-previous.

Full cargo test suite green; cargo clippy clean (only a pre-existing unrelated warning). Smoke-tested against --null: first boot logs "no last-session config yet", writes the file on shutdown, second boot logs "loaded startup config '_last_session'". Docs updated in saving-loading.md and default-rig-config.toml.

🤖 Generated with Claude Code