fix(record): remove duplicated header written by StandardVariableRecord.serialize

[ngjunsiang]

Problem: StandardVariableRecord.serialize has a concrete abstract-method body that writes recordType (u32) + recordLength (u32), while every concrete subclass writes its own recordType u32 + recordLength u16 header after calling super().serialize() — so serialized SV records carry 8 duplicated header bytes and recordLength under-reports the record size (e.g. DirectedEnergyDamage: marshalledSize() 40, serialize() emits 48), and parse — which expects the single 6-byte header — misaligns on self-produced output.

Fix: reduce StandardVariableRecord.serialize to a docstring-only abstract method so subclasses own the §6.2.83 header; no production behavior other than removing the duplicated bytes.

Fixes #89.

Adds regression test tests/test_StandardVariableRecord.py (serialized length == marshalledSize == recordLength).