refactor(runner): group route execution state

[afourniernv]

What

• Groups the algorithm, clients, runtime models, auxiliary targets, and decision targets in a private ExecutionLane.
• Keeps caller authentication and advertised capabilities on Route.

Why

This is the first internal step toward letting a route choose between complete standard and restricted execution lanes. Keeping the execution state together avoids threading parallel sets of clients and models through the runner later.

Linear: SWITCH-1620

Notes for reviewers

This is a behavior-preserving refactor. It does not change public APIs, TOML, ownership, or per-request work.

Validation:

• cargo fmt --all --check
• cargo clippy -p switchyard-runner --all-targets --all-features -- -D warnings
• cargo test -p switchyard-runner --all-features
• cargo test -p switchyard-server --all-features

[afourniernv]

Opened this too early. Closing while the design and implementation sequence are still being worked through.