docs(foundation): extraction.py's engine-check comment cites an alias #750 removed

[JarryShaw]

Describe the bug

pcapkit/foundation/extraction.py:430-432 explains its issubclass(engine, EngineBase) check by citing a
code shape that no longer exists. #750 removed the alias it names.

Reproduction

The comment, on 73f09aecb:

# NOTE: checked against ``EngineBase`` rather than ``Engine``: every built-in
# engine subclasses the base directly (``engines/pcap.py`` imports it as
# ``EngineBase as Engine``) precisely so that it is *not* auto-registered by

But pcapkit/foundation/engines/pcap.py:13 reads from pcapkit.foundation.engines.engine import EngineBase, and :29 is class PCAP(EngineBase[Frame]). There is no as Engine anywhere in that file.

Expected behavior

The comment should cite the shape that is actually there. Its conclusion is still right — built-in
engines do subclass the base directly, which is why the check is against EngineBase — so only the
parenthetical is wrong.

System information

• OS Version: Linux 5.10 (Amazon Linux 2 int)
• Python Version: 3.14.7
• Python Implementation: CPython
• pcapkit Version: checkout at 73f09aecb

Additional context

Found while cross-reviewing #752. It is the Engine family, not the ProtocolBase family #752 touches,
and sits ~370 lines from that PR's nearest edit, so it was correctly left out rather than folded in.

Non-urgent: a comment, not logic, so nothing behaves wrongly. Worth fixing because it points a reader at
an alias that was deliberately removed — the same class of defect as a stale line-number citation, and #657
has spent eighteen review rounds on exactly that.