refactor(parser): promote instead-override SpecialClause variants to ClauseDisposition::ReplaceMeaning (U5-M2)

[matthewevans]

Fifth U5-M2 increment. DigInsteadAlt + InsteadClause + KeywordInsteadOverride
(CR 608.2c / CR 614.1a / CR 608.2e — all intrinsic: None) share the structural
role "replace/override the meaning of the prior emitted def(s), emit no sibling"
and fold into ClauseDisposition::ReplaceMeaning { kind: ReplaceMeaningKind },
where kind (DigAlt | Instead | KeywordOverride) carries each variant's payload
and keeps the distinct rules concept typed. AdditionalCostInsteadSearch
(intrinsic-carrier) and DrawnThisTurnPayOrTopdeck are deliberately left for inc6.

The three lower.rs handler bodies — including InsteadClause's intricate multi-clause
Cow-swap (mem::take the def chain, build the base, stash the tail in the override's
else_ability) — move byte-for-byte under match kind; only the arm pattern names
change (the else-if block evaluates to the match's bool, so the per-arm true
bodies are untouched). All three SpecialClause variants are deleted.

Faithful-by-construction (ClauseDisposition never reaches a serialized snapshot).
All three kinds have real fixture cards (DigAlt 2, Instead 24, KeywordOverride 1);
parity: Follow the Lumarets (DigAlt — else_ability + Dig), Evil's Thrall (Instead —
ConditionInstead override + Untap/Haste tail in else_ability), Conformer Shuriken
(KeywordOverride — sub_ability + counters). Zero snapshot drift.

[gemini-code-assist]

Code Review

This pull request refactors the representation of meaning-replacement overrides in the parser by introducing the ReplaceMeaningKind enum and ClauseDisposition::ReplaceMeaning variant, replacing several older SpecialClause variants. It also adds corresponding snapshot tests. The reviewer pointed out a violation of Rule R6 in lower.rs, where the KeywordOverride match arm is missing a mandatory CR 608.2e annotation comment.

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[gemini-code-assist]

According to the repository style guide (Rule R6), every rules-touching line of engine code must carry a verified CR <number> annotation. Please add the CR 608.2e annotation to the KeywordOverride arm comment.

Suggested change

	ReplaceMeaningKind::KeywordOverride => {
	// Build the def for this clause and attach to previous as sub_ability
	ReplaceMeaningKind::KeywordOverride => {
	// CR 608.2e: Build the def for this clause and attach to previous as sub_ability

References

1. Every rules-touching line of engine code must carry a comment of the form CR <number>: <description> (regex CR \d{3}(\.\d+[a-z]?)?). ^(link)

[github-actions]

Parse changes introduced by this PR

✓ No card-parse changes detected.