register_extractor_engine/reassembly/traceflow reject pcapkit's own built-in classes

[JarryShaw]

All three public registration entry points refuse the library's own classes. Reproduced in a clean process against 27bb315d5:

register_extractor_engine('zzz_e', 'pcapkit.foundation.engines.pcap', 'PCAP')
  -> RegistryError: engine must be an Engine subclass, not <class '...pcap.PCAP'>
register_extractor_reassembly('zzz_r', 'pcapkit.foundation.reassembly.ipv4', 'IPv4')
  -> RegistryError: reassembly must be a Reassembly subclass, not <class '...ipv4.IPv4'>
register_extractor_traceflow('zzz_t', 'pcapkit.foundation.traceflow.tcp', 'TCP')
  -> RegistryError: traceflow must be a TraceFlow subclass, not <class '...tcp.TCP'>

Cause

The validations at pcapkit/foundation/extraction.py:390, :412 and :434 test issubclass(x, Engine | Reassembly | TraceFlow) — the auto-registering public classes. But every built-in subclasses the *Base variant, imported under a local alias:

• pcapkit/foundation/engines/pcap.py:13 — from ...engine import EngineBase as Engine
• pcapkit/foundation/reassembly/ip.py:22 — ReassemblyBase as Reassembly
• pcapkit/foundation/traceflow/tcp.py:16 — TraceFlowBase as TraceFlow

Measured:

issubclass(PCAP, Engine)     : False
issubclass(PCAP, EngineBase) : True

Descendant counts across the whole package make it clear this is universal, not incidental:

public class	descendants	base class	descendants
Protocol	0	ProtocolBase	43
Engine	0	EngineBase	9
Reassembly	0	ReassemblyBase	5
TraceFlow	0	TraceFlowBase	2

The aliasing is deliberate — subclassing the public class would auto-register the built-ins via __init_subclass__ — but it makes the validation stricter than the type hints and docstrings promise.

Suggested fix

Name EngineBase / ReassemblyBase / TraceFlowBase in those three issubclass checks. That accepts both the built-ins and any external subclass (a Protocol subclass is also a ProtocolBase), so it only widens.

Related: the same public-vs-base confusion left a branch dead for three years in #506 (schema/schema.py checked isinstance(data, Protocol) where it meant ProtocolBase), and the underlying design question is filed separately.