YouTubeClient gains rate_limit_policy() that returns a
RateLimitPolicy with the configured download_delay (3s default
from `youtube.download_delay`). Engine reads this at
register_plugin time + applies to engine.worker.
set_engine still re-applies the delay so runtime reload_settings
updates flow through the same pathway. Other sources keep the
default policy (concurrency=1, delay=0) which matches their
current behavior — no migration needed beyond YouTube which is
the only source with a non-default download throttle today.
New pinning test asserts the policy shape (delay=3.0, concurrency=1).
Suite still green (2042 passed).
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