docs: say where async waits with no worker

[cardmagic]

The Ruby counterpart of
solid-objects-js#27,
which closes
solid-objects-js#22.

Why

The JS issue came from an external prober who built a two-process harness from
the README, watched ready messages sit unclaimed for 30 seconds, and concluded
messages were stranded. Nothing was wrong with the runtime. The page never said
that installing starts no role.

The gem is in better shape than the Node package was. ## Worker requirements
already carries the feature-by-role table and the exact sentence a worried
reader needs:

A missing worker never makes a durable async message disappear, but it
leaves the message pending indefinitely.

What was missing is the pointer at the point of use. A reader who jumps to
### async from the table of contents reads only that execution is "left to
the worker fleet," which does not tell them a fleet has to exist, or what
happens meanwhile.

What changed

README, in the async section:

async needs a running actor worker. Installing the engine and migrating the
schema starts no role, so a process that only serves web requests leaves the
message ready. Nothing is lost. The message waits until
bundle exec solid_objects start runs the roles. See Worker requirements for
the feature-by-role table.

docs/operations.md, in the runtime section: the same claim, plus the
boundary that makes it make sense. A direct call or an explicit sync needs no
running role, because the caller's own path executes it.

Keeping the sentence honest

test/integration/background_pickup_test.rb pins the documented behavior:

message = MailboxActor.ref("inbox").async.receive

assert_equal "ready", message.status
assert_empty SolidObjects::Instance.find_by!(
  actor_type: "background-pickup-mailbox", actor_id: "inbox"
).state

worker = SolidObjects::Worker.new
worker.run_until_idle

assert_equal "completed", message.status

A test that only ever sees ready proves nothing, so I checked that the first
assertion detects pickup. Running a worker before it turns the test red:

Failure:
BackgroundPickupTest#test_leaves_an_async_message_ready_until_a_worker_runs_the_roles:
Expected: "ready"
  Actual: "completed"

One detail worth recording, because it surprised me while writing the test: the
enqueue does create the instance row, since it locks the instance to allocate
the mailbox sequence. So "nothing ran yet" shows up as empty state, not as a
missing row. The first draft asserted a missing row and failed:

Expected #<SolidObjects::Instance id: 1, actor_type: "background-pickup-mailbox",
actor_id: "inbox", state: {}, state_version: 1, next_message_sequence: 2, ...> to be nil.

Effects

• API: none.
• Correctness: none. The test asserts existing behavior.
• Security, migration, compatibility: none.

docs/roadmap.md is untouched, per AGENTS.md: this edits prose and adds a test
rather than moving what the project claims about itself.

Validation

• bundle exec rake (test, standard, rubocop, rbs, steep, security): pass.
• Mutation check on the new test, quoted above.