refactor(control-plane): resolve turn result kind through effect turn

[huangruiteng]

Summary

R2 replacement-first change: turn executor resolves result kind through
EffectTurn.

• _task_validation_stage now reads interpret_turn_result_packet(...) to
  derive LoopXTurnResultKind, replacing direct result["result_kind"]
  access in the real turn-driver runtime path.
• Add a focused test proving the stop-result path still routes through the
  effect turn lens.

No public behavior changes.

Validation

• python -m pytest tests/test_loopx_turn_executor.py tests/control_plane/test_effect_turn_turn_result.py: 20 passed.
• examples/loopx-turn-fake-host-walkthrough-smoke.py: passed.
• cli-output-budget-regression-smoke and
  control-plane-risk-characterization-smoke: passed.
• loopx canary premerge --from-git-diff: passed, self_merge_allowed=true,
  0 manual holds.
• Public boundary scan: passed.

Routing

• continuation-policy: independent_handoff
• excluded-agent: codex-quality-qualification
• claimed-by: codex-side-bypass

[huangruiteng]

Approval conclusion (author-owned PR; GitHub blocks formal self-approval)

Reviewed exact head 9b59876. No blocking findings.

The PR routes the turn executor's task-validation result-kind resolution
through interpret_turn_result_packet, replacing direct dict access in a real
runtime path. Public behavior unchanged.

Validation

• python -m pytest tests/test_loopx_turn_executor.py tests/control_plane/test_effect_turn_turn_result.py: 20 passed.
• examples/loopx-turn-fake-host-walkthrough-smoke.py, CLI output budget, and
  control-plane risk characterization: passed.
• loopx canary premerge --from-git-diff: passed, self_merge_allowed=true,
  0 manual holds.
• GitHub pytest, build, and dependency-review on this head: pass.
• Public boundary scan: passed.

Merge decision: approved after reviewer.