Lower guarded equations, guarded case alternatives, and multi-scrutinee case #88

Part of #75 (Phase 2). 12 passing files stop on guards and multi-scrutinee case, and two more M2 failing cases (LetPatterns2.purs, OverlappingBinders.purs) depend on the same machinery.

Scope

• Guarded equations — f x | p = e (5 files: passing/2787.purs, passing/2806.purs, passing/DctorName.purs, passing/FunctionAndCaseGuards.purs, passing/TCO.purs).
• Guarded case alternatives — alt | p -> e (5 files: passing/2795.purs, passing/3114/VendoredVariant.purs, passing/4357.purs, passing/Guards.purs, passing/MultiArgFunctions.purs).
• Comma-separated guard lists — alt | a, b -> e. The parser already accepts these (parse_guard_list skips Comma at crates/psrs-syntax/src/parser/declaration/mod.rs:137), so this is lowering only.
• let and pattern guards — | Just x <- action, | let y = e.
• Multiple scrutinees — case a, b of (passing/CaseMultipleExpressions.purs, passing/CaseInputWildcard.purs).

Current diagnostic split

The AST layer reports distinct messages — "guarded case alternatives are not supported yet" and "guarded equations are not supported yet" — so the blocker is localized and the two forms are tracked separately today.

Design note

A guard is a filter on an alternative, and case a, b of is a tuple scrutinee plus per-alternative pattern matching. Both reduce cleanly onto the existing decision DAG: guards become additional decision rows that must all succeed, and multiple scrutinees become a shared scrutinee representation. Neither should need a new CC or MIR form — which is why this belongs before Phase 5 rather than in it.

Acceptance

Both guard forms lower with comma lists, let/pattern guards, and multi-scrutinee case; exhaustiveness and redundancy analysis account for guards.