Remove dead code from Monomorphizer.zig

The Monomorphizer module was never integrated into the compiler - it was
only used in tests. This commit removes:

- 2400+ lines of unintegrated code including:
  - setClosureTransformer, lookupStaticDispatch, getConcreteTypeName
  - resolveUnspecializedClosures and related resolution functions
  - registerPartialProc, isPartialProc
  - requestSpecialization, requestExternalSpecialization
  - processPendingSpecializations and the entire specialization pipeline
  - All duplicate* functions for body/expr/pattern duplication
  - isPolymorphic, isTagUnion, getTagNames, countTags
  - createSpecializedName, getSpecializationCount, getSpecializedName
  - specializedIterator

- Tests that only tested the dead code
- cross_module_mono_test.zig which tested the removed functions

What remains:
- Core types and data structures for future integration
- init/deinit for lifecycle management
- structuralTypeHash for type hashing (useful utility)
- Basic tests for the remaining functionality

Coverage improved from 81.4% to 85.33% by removing untested dead code.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: rtfeldman