Lower multi-field, nested-record, literal and binder patterns #85

Part of #75 (Phase 2). 41-46 passing files stop in pattern lowering. Measured first-failure forms, with representatives:

1. Multi-field constructor patterns — Person name true -> name (passing/1185.purs). Single-field constructor patterns work; more than one field does not.
2. Record patterns nested in a constructor pattern — f ( r@{ x } : _ ) = x + r.y (passing/2049.purs).
3. Pattern bindings — patternNewtype = ... (passing/LetPattern.purs), and the DuplicateDeclarationsInLet2/3 and OverlappingBinders failing cases.
4. Pattern binders — BindersInFunctions.purs, NegativeBinder.purs, TypedBinders.purs, ParensInTypedBinder.purs.
5. Literal, array, and named patterns (passing/IntAndChar.purs, passing/NamedPatterns.purs, passing/Patterns.purs).
6. Operator patterns — passing/ExtendedInfixOperators.purs, passing/MinusConstructor.purs.

Dependencies

Operator and constructor-operator aliases (see the declaration-alias slice) must land first for (6): infixr 6 Cons as : is a declaration our lowering rejects outright.

Pattern bindings and binders also unblock three M2 failing cases that currently report no diagnostic: DuplicateDeclarationsInLet2.purs, DuplicateDeclarationsInLet3.purs, OverlappingBinders.purs.

Note on the backend

PM-01..PM-13 are Verified and the CC/MIR rows are Partial — not because the decision compilation is weak, but because source-path coverage is narrow. Items 1 and 2 are the cases where "a multi-field constructor pattern" and "a record inside a constructor pattern" reach a coverage analysis that has only seen single-field patterns. Landing them is expected to surface backend work, which is Phase 5's input.

Acceptance

Every file in the measured pattern-lowering blocker list lowers; docs/implementation/backend/pattern-matching.md gains source-path evidence for multi-field and nested-record matrices.