Laurel: add constrained type support

[fabiomadge]

Adds constrained types to Laurel via a Laurel-to-Laurel elimination pass that inserts verification checks at type boundaries.

Syntax

constrained nat = x: int where x >= 0 witness 0
constrained posnat = x: nat where x > 0 witness 1

How it works

The pass inserts checks where constrained types cross boundaries:

• Receiving a value (procedure inputs, call return values): assume the constraint
• Providing a value (assignments, arguments, return values): assert the constraint

All constrained type references are then resolved to their base types. The Core translator sees only base types and regular assert/assume — no translator changes needed.

Changes

• ConstrainedTypeElim.lean — the elimination pass
• LaurelGrammar.st — constrained type syntax
• ConcreteToAbstractTreeTranslator.lean — parser
• LaurelToCoreTranslator.lean — pipeline wiring (2 lines)
• T09_ConstrainedTypes.lean — 11 test procedures

Known limitations

• Witness is parsed but not used (we don't rely on constrained types being inhabited)
• Multi-target assignments not yet handled
• Quantifier constraint injection is a follow-up

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