Refactor runtime scenario execution into explicit stage modules

[danielbdyer]

Motivation

• Improve separation of concerns in runtime scenario execution by splitting handshake, interpretation, route selection, step execution, recovery, and receipt-finalization into dedicated modules.
• Make route-selection pure and deterministic to protect precedence behavior and make scoring easier to test.
• Restrict ephemeral run-local mutation to a single coordinator to reduce surface area for side effects and make stage contracts explicit.

Description

• Added lib/runtime/scenario/types.ts to define stage input/output contracts, stage envelopes, lane metadata, and a small ScenarioContextRef type using readonly records where applicable.
• Extracted route-selection logic (normalization, variantStateMatchScore, routeVariantsForScreen, and selectRouteForNavigate) into lib/runtime/scenario/route-selection.ts as pure functions.
• Implemented stage modules: handshake.ts, interpretation.ts, step-execution.ts, recovery.ts, and receipt-finalization.ts under lib/runtime/scenario/ and delegated implementation to them.
• Kept a thin runScenarioStep coordinator in lib/runtime/scenario.ts which composes the stages in order, is the sole place mutating run-local ScenarioRunState, and preserves public exports (runScenarioStep, runScenarioHandshake, createScenarioRunState, stepHandshakeFromPlan).
• Added stage-level tests tests/runtime-scenario-stages.spec.ts to lock deterministic route-choice and recovery transition ordering using fixed fixtures.

Testing

• Ran npx playwright test tests/runtime-scenario-stages.spec.ts and the two stage-level tests passed.
• Ran npm run types which failed due to existing type issues outside the refactor surface (e.g. lib/playwright/state-topology.ts).
• Ran npx playwright test tests/runtime-agent-pipeline.spec.ts which surfaced 3 existing pipeline-level failures (precedence/fixture and DOM-mock shape issues) that are not introduced by this refactor.

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Codex Task