v4: Unify negation rules into single NegationRule with strategy enum

[jtdub]

Description

The current negation system has three separate rule types and an implicit priority chain in negate():

1. NegationDefaultWithRule (replace with fixed string)
2. NegationDefaultWhenRule (use default form)
3. NegationSubRule (regex substitution)
4. swap_negation() (fallback)

These are three different Pydantic models, three fields on HConfigDriverRules, and the precedence is encoded in method ordering rather than being explicit.

Proposed Change

Unify into a single rule type:

class NegationStrategy(Enum):
    REPLACE = auto()      # was negate_with
    DEFAULT = auto()      # was negation_default_when
    REGEX_SUB = auto()    # was negation_sub

class NegationRule(BaseModel):
    match_rules: tuple[MatchRule, ...]
    strategy: NegationStrategy
    value: str = ""       # replacement text or regex pattern
    replace: str = ""     # for REGEX_SUB strategy

Single negation_rules field on HConfigDriverRules. First matching rule wins.

Benefits

• Collapses three rule types and three driver fields into one
• Precedence is explicit (list order) rather than implicit (method ordering)
• Easier to understand and extend

Breaking Change

Yes — all existing negate_with, negation_default_when, and negation_sub rules must be migrated.