Add initial draft of Pact type system formal specification

[colinrozzi]

Motivation

• Introduce a precise, implementer-facing formalization of Pact's type system to guide type checker, trait, and interface design.
• Capture decisions and open questions around parametric polymorphism, trait-constrained generics, interface transforms, and ABI compatibility.

Description

• Add a new draft specification file at docs/spec/TYPE-SYSTEM-SPEC-DRAFT.md that defines the core semantic model, syntactic categories, and an initial kind system.
• Specify static semantics with judgments for well-formed types, type declarations, trait declarations, and interface declarations, along with an HM-style constraint/inference outline.
• Propose a candidate coherence rule for trait implementations and describe interfaces as algebraic objects with union/extension operations and canonical normalization rules.
• Define transforms as typed, terminating rewrites, state ABI boundary rules, list soundness targets, and enumerate open questions and a proposed PR sequence toward normative rules.

Testing

• No automated tests were added or executed for this documentation-only draft.

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Codex Task