v4: Make drivers declarative-only (no method overrides)

[jtdub]

Description

Currently drivers can extend behavior two ways: declarative rules (via HConfigDriverRules) and imperative method overrides (idempotent_for(), swap_negation(), config_preprocessor()). This dual mechanism makes behavior harder to predict and prevents drivers from being fully serializable.

Proposed Change

Move all imperative overrides into the rule system:

• XR's ACL sequence number logic → IdempotentKeyExtractorRule or similar
• FortiOS/VyOS/Juniper swap_negation() → unified negation rule system (v4: Unify negation rules into single NegationRule with strategy enum #220)
• config_preprocessor() → FullTextSubRule or new PreprocessorRule

Benefits

• All driver behavior lives in HConfigDriverRules
• Drivers become fully serializable (could be loaded from YAML/JSON)
• Easier to test — no method mocking needed, just rule configuration
• Users can extend drivers without subclassing

Breaking Change

Yes — all drivers that override methods must be rewritten to use rules.