refactor: add physical endpoint label plumbing

[okhomenko]

Summary

This PR adds preparatory plumbing for physical endpoint labels without changing the default UI behavior yet.

It establishes a shared PipeWire-state layer for endpoint/profile label lookup and exposes a computed naming key that follow-up PRs can start using for endpoint defaults and Configuration titles.

What changed

• parse and store raw SPA_PARAM_ROUTE_info entries on EnumRoute
• add shared state helpers for:
  • active route lookup for endpoint nodes
  • physical endpoint label lookup for endpoint nodes
  • physical endpoint label lookup for device profiles
• add a computed: namespace to the naming/matching system
• implement computed:physical.endpoint.label
• refactor view.rs to use shared state helpers for route/profile/device matching instead of owning that logic locally
• document the new computed key in README.md and wiremix.toml

Behavior

computed:physical.endpoint.label resolves in this order:

1. node.nick
2. the active route's device.product.name when it maps unambiguously
3. unresolved

This PR does not change the default endpoint naming templates or Configuration titles yet.

Follow-ups

• switch default endpoint naming to prefer {computed:physical.endpoint.label}
• reuse the same plumbing to enrich Configuration profile titles and selected-profile text
• expand docs/examples for template overrides using the computed key

Testing

I added unit coverage for route-info parsing, computed-key rendering, computed-key override matching, and ambiguous profile-label cases.