feat: align replay snapshots with compose descriptors

[ForeverAngry]

Summary

rig-mcp now consumes the descriptor surface released in rig-compose 0.4.1 instead of treating schema discovery as a storage-specific registry detail.

Loopback discovery and registry-backed replay snapshots now flow through ToolRegistry::descriptors(), so MCP registration replay uses the same deterministic catalog contract as local compose callers.

Motivation

The previous replay slice intentionally stayed compatible with rig-compose 0.4.0, which meant RegistrationSnapshot could only talk in raw ToolSchema terms. After rig-compose 0.4.1 shipped descriptor snapshots, rig-mcp can close that gap without adding a path dependency or moving reconnect state into the kernel.

Design

• Raise the rig-compose dependency floor to 0.4.1.
• Have LoopbackTransport::list_tools read from ToolRegistry::descriptors().
• Add RegistrationSnapshot::from_registry and from_registry_with_policy for host-owned reconnect flows that already have a scoped registry view.
• Keep replay state adapter-owned; ToolRegistry remains a catalog/execution surface, not a transport lifecycle store.

Behavioural changes

• Loopback MCP discovery now uses the published descriptor contract.
• Registry-backed snapshots honour ToolRegistry::scoped visibility through the descriptor surface.
• Existing transport discovery and replay APIs remain additive/backward-compatible.

Tests

• just check
• Added unit coverage for RegistrationSnapshot::from_registry descriptor visibility and scoped-registry behaviour.

Out of scope

• Transport-specific reconnect loops, heartbeat policy, and in-flight call recovery remain host/adapter concerns.
• Cached result auto-wrappers and page/search/release tools remain separate result-cache follow-up work.