engine+cli: introspection + step-by-step execution (§14)

[fruwe]

Closes #21.

Implements introspection + step-by-step execution (SPEC §14) in the engine and CLI.

Library (Host/Instance):

• inject(instance, event, payload=None) — validate + enqueue, no processing (in either
  mode); deliver now delegates to it.
• step(instance, n=1) — process exactly n RTC steps; returns one per-step record each
  { event, transition, entered, exited, published, spawned, faulted }. Deltas for
  entered/exited come from the active_leaf_names() diff; published/spawned from
  the host deltas; transition is the target state name (captured in run_transition).
• inspect(instance) — { status, config, esvs, queue, deferred, timers, history, dead_letter }.
• a mode flag (auto|manual) on the host/store; maybe_run() runs to quiescence in auto
  mode only, so manual mode enqueues without processing.

CLI: harel mode [auto|manual] (persisted in the store), harel inject,
harel step [--steps N], harel inspect [--json]. In manual mode send/env/advance
enqueue/arm without running. mode is carried through the store's meta.json.

Tests: unit tests for inject/step/inspect/mode + the auto↔manual toggle, and the new
cli/03-stepping black-box case (suite fetched from fruwehq/harel-conformance@main
until the v0.0.1 tag exists). ruff + mypy clean; 152 tests pass.