feat: add the experimental native ECS runtime (rig-ecs)

[gold-silver-copper]

Summary

Adds rig-ecs, an experimental, native-only ECS-based agent runtime built
on the third-party bevy_ecs engine. Opt in with the ecs feature and use it
through rig::ecs; the classic runtime remains the default.

Stacked PR. This targets split/core-agent (#2197), not main. Merge
after #2197. The diff shown here is only the ECS layer.

Design

• The ECS world is authoritative for agent topology and run progression;
  provider/tool/memory work is modeled as owned effects that re-enter through
  validated ingress (generation + correlation + authorization checks).
• Construction uses EcsClientExt::ecs_agent, distinct from the classic
  CompletionClient::agent, so both runtimes can be used together without
  method collisions.
• rig-ecs depends only on rig-core (never rig-agent); the dependency-graph
  guard enforces it. The shared rig-runtime-conformance ledger holds both
  runtimes to the same observable behavior.

Notes

• Native-only: it compile_error!s on wasm and is absent from the facade wasm
  feature; it is not in default features.