Harden winn_transform.erl pass ordering and add interaction tests

[gregwinn]

Summary

winn_transform.erl is the most complex file in the compiler at 689 lines, running 7 ordered desugaring passes. Each pass assumes specific invariants from prior passes (e.g., multi-clause merge depends on case-wrapping, import rewriting runs last). As features interact in new ways, subtle ordering bugs can emerge.

Proposed Work

• Add explicit integration tests that exercise pass interactions, not just individual features:
  • Import + pipe + pattern matching combined
  • Schema + protocol + default params on the same module
  • Use directive + multi-clause merge + guard functions
  • Struct + impl + import rewriting
• Document the invariants each pass expects from prior passes (comments or a design doc)
• Consider adding an assertion/validation step between passes during debug builds to catch invariant violations early

Why

This is the file where new features are most likely to introduce regressions. The ordered-pass design is good — but the ordering contract is implicit. Making it explicit and well-tested prevents future "works in isolation, breaks in combination" bugs.

Target Version

v0.10.0 (Hardening)