fix(parser): lower 'gains no life instead' to Prevent replacement (#743)

[matthewevans]

Summary

Fixes #743 — Sulfuric Vortex's second ability ("If a player would gain life, that player gains no life instead") was silently dropped. (The damage half was already correct, structurally identical to Roiling Vortex.)

Root cause

The "would gain life … instead" replacement clause was routed through parse_effect_chain, which lowered "gain no life" to Effect::Unimplemented. The engine treats Unimplemented as passthrough → no substitution → the life gain proceeded.

Fix (parser-only)

In the existing "would gain life … instead" branch of oracle_replacement.rs, detect the lifegain-negation body with nom combinators (value(Prevent, alt((tag("gains no life"), tag("gain no life")))) over the lowercased body) and emit .quantity_modification(QuantityModification::Prevent) with no execute, mirroring parse_global_player_counter_prohibition. The engine's gain_life_applier already reads quantity_modification first and Prevent => ApplyResult::Prevented fully suppresses the gain (CR 614.6) — no engine change needed.

Scope (corrected from the plan)

The genuine "would gain life … gains no life instead" replacement class is exactly 2 cards: Sulfuric Vortex and Flames of the Blood Hand. (The Erebos / Everlasting Torment / Leyline of Punishment "can't gain life" cards are a separate, already-handled StaticAbilityMode::CantGainLife static class.) Flames carries a this turn duration (CR 611.2a) and is explicitly guarded out of the permanent Prevent (!scan_contains(lower, "would gain life this turn")) so it keeps its prior deferred behavior — its durational variant is a follow-up.

Tests

• sulfuric_vortex_prevents_lifegain (runtime): casts a real "gain 3 life" spell with Sulfuric Vortex out, asserts life unchanged; paired control proves the spell gains life without the Vortex.
• Parser building-block assertion: lowers to quantity_modification: Prevent, not Unimplemented.
• Negative guards: a lifegain doubler must not collapse to Prevent; Flames must not carry a permanent Prevent.

Plan reviewed clean + implementation reviewed clean (independent reviewer confirmed bug-is-fixed, nom-compliance, and Flames scoping). CR numbers grep-verified (119.10, 614.6, 614.1a, 611.2a).

[gemini-code-assist]

Code Review

This pull request implements parsing and execution support for lifegain-negation replacement effects (such as Sulfuric Vortex's "gains no life instead") by mapping them to a structured QuantityModification::Prevent instead of an Unimplemented no-op. It also refactors player scope parsing for lifegain replacements into a shared helper and adds comprehensive unit and integration tests. The feedback suggests wrapping the lifegain-negation parser combinator in all_consuming to prevent partial matches on compound effects.

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[gemini-code-assist]

Using all_consuming ensures that the entire effect body is matched. Without it, a compound effect starting with a lifegain negation (e.g., "gains no life and draws a card instead") could be partially matched, resulting in the execute effect being silently dropped.

Suggested change

	let mut combinator = preceded(
	subject,
	value((), alt((tag("gains no life"), tag("gain no life")))),
	);
	combinator.parse(lower_body.trim()).is_ok()
	let mut combinator = all_consuming(preceded(
	subject,
	value((), alt((tag("gains no life"), tag("gain no life")))),
	));
	combinator.parse(lower_body.trim()).is_ok()

References

1. Avoid verbatim string equality for parsing Oracle phrases as it bypasses the robust nom-based parser and creates fragile matches. Instead, decompose compound phrases into modular, reusable parsers for constituent parts and compose them using idiomatic combinator aggregates to prevent combinatorial explosion and improve maintainability.