feat(web): InteractiveSession — threadless stepwise play driver

refactor(web): SessionRunner delegates to _session_core (no behavior change)

test(web): lock finalize SessionNotFinishedError divergence + docstring note (Task 1 follow-up)

feat(web): extract shared session-core helpers (build/turn/finalize)

feat(cp6): runtime.aggregate — pure (model,difficulty) trace aggregation

feat(cp6): trajectory + category-reward metrics (away_move_fraction, trajectory_agreement, ...)

feat(cp6): runtime.rollout — deterministic optimal-trajectory simulator

feat(cp6): scenario-owned survival step_reward (away-from-predator) + safety_distance

feat(cp4.5): preserve probe reasoning + raw_text + token accounting via ProbeResult

feat(cp4.5): persist act-call token accounting + raw_text in TurnTrace

feat(cp4): JSONL trace persistence (append_trace / read_traces)

feat(cp3): runtime re-exports + full-session golden integration test

feat(cp3): guard cut-replay termination + cover survived/eliminated/cut-frame contracts

feat(cp3): SessionRunner orchestrator (cut replay -> play -> trace)

test(cp3): cover perfect/all-diagnostic/cap/empty metrics + single-source divergence

feat(cp3): session metrics (motive-reading, reactivity, survival, divergence)

feat(cp3): constrain outcome to Literal, default seed, test nested trace roundtrip

feat(cp3): lean TurnTrace + SessionTrace models