fix(px): when+return short-circuits procedure execution

[Copilot]

After extracting the px runtime to pluresdb-px, return inside a when: block no longer halted procedure execution — subsequent steps continued running regardless.

Root causes

executor.rs — execute_when iterated nested steps without checking for a return result, and always returned kind: "when". The outer procedure loop's is_return guard never fired.

scenario_runner.rs had a hand-rolled execute_step loop with no "return" arm and no short-circuit check.

Changes

• executor.rs execute_when: break on the first nested return, propagate kind: "return" to caller so the procedure loop short-circuits correctly. Works for arbitrarily nested when blocks.

• scenario_runner.rs: replace the hand-rolled per-step loop with execute_with_vars (which already handles is_return). The executor's emit step stores events in vars["emit"] rather than calling the handler, so emitted events are replayed through handler.call("emit", …) post-execution so ScenarioActionHandler captures them for event_emitted checks. Replay errors propagate as scenario failures.

procedure guard_test:
  trigger: manual
  when $input == "":
    return "no_input"   # ← now correctly halts; steps below no longer run
  end
  return $input

Tests added

File	Test	Covers
executor.rs	when_return_short_circuits_procedure	condition true → single return step result
executor.rs	when_condition_false_does_not_short_circuit	condition false → execution continues
executor.rs	when_return_nested_when_also_short_circuits	return propagates through two when levels
scenario_runner.rs	when_return_short_circuits_procedure_in_scenario	entry not stored, var bound to guard return value
scenario_runner.rs	when_condition_false_continues_execution	guard miss → subsequent steps run