provider: add Waypipe-backed remote WaylandSession

[vicondoa]

Problem

v3 does not currently model Waypipe as the Provider implementation for remote
WaylandSession cases. U14 retires the ADR45 gateway compatibility surface,
including the old gateway-owned Waypipe argv and systemd planning, but the
remote Wayland use case still needs a v3 owner.

The retired implementation is preserved in Git history here:

• packages/d2b-gateway-runtime/src/waypipe_display.rs at 97f177d0f

That code is a reuse/characterization source, not an authority to restore the
old realm-provider framework.

Scope

• Define a v3 Provider-owned Waypipe implementation for remote
  WaylandSession resources.
• Put guest Waypipe server planning, gateway/host client planning, argv,
  readiness, and lifecycle under the owning display Provider rather than
  d2b-gateway-runtime compatibility modules.
• Bind the implementation through current Zone/session, Provider, Process, and
  Endpoint contracts; do not reintroduce d2b-realm-provider,
  d2b-realm-router, or generic ADR45 provider traits.
• Preserve realm boundaries: credentials and remote relay configuration stay
  in the per-realm gateway guest, and work/personal realms do not share a
  gateway or L2 bridge.
• Decide whether the existing d2b-provider-display-wayland crate owns a
  Waypipe component/implementation or whether a dedicated
  d2b-provider-display-waypipe leaf gives the narrower dependency graph.
• Add owner-local Rust and Nix coverage for remote session admission, argv,
  readiness, disconnect/cleanup, and redacted status.
• Update Provider catalog/schema, WaylandSession documentation, and changelog
  together.

Acceptance criteria

• A remote WaylandSession selects a declared Waypipe Provider implementation
  and reaches the gateway guest without host-held realm credentials.
• The Provider owns both ends' process plans and reports typed readiness and
  failure state.
• Session teardown cancels/cleans both Waypipe processes without broad host
  cleanup or cross-realm effects.
• Local Wayland display behavior remains unchanged.
• d2bd-runtime and d2b-guestd remain free of individual Provider
  implementation dependencies.
• Focused Provider Rust/Nix tests and bare make check pass.

[vicondoa]

Position: kept open and explicitly deprioritised, so it reads as a wanted-but-unstarted direction rather than a commitment. The only remote-display work in the tree is a proof of concept deliberately kept outside the workspace.