feat(extensions): resolve UDT URNs while parsing

[benbellick]

Draft alternative to the stacked dependency/UDT validation work.

This explores resolving user-defined type references while parsing extension YAML type expressions instead of constructing unresolved u!name nodes and inferring their URN later from the function variant.

Key changes:

• adds parser options with a UserDefinedTypeResolver
• stores the resolved URN on types.ParameterizedUserDefinedType
• parses extension YAML function signatures with resolver context from the current file's urn and types: declarations
• rejects undeclared local UDT references during extension YAML parsing
• uses the resolved UDT URN when assigning type anchors

This intentionally starts with local UDT resolution. Dependency-qualified references can build on the same resolver shape by resolving aliases to dependency URNs.

Tests:

• go test ./...

---

Note: This PR was developed with AI assistance. All changes have been reviewed, and I take full responsibility for this contribution.

[codecov]

Codecov Report

❌ Patch coverage is 77.65957% with 21 lines in your changes missing coverage. Please review.
✅ Project coverage is 69.02%. Comparing base (5a03350) to head (88f5cc0).

Files with missing lines	Patch %	Lines
extensions/simple_extension.go	83.58%	6 Missing and 5 partials ⚠️
types/parser/visitor.go	25.00%	4 Missing and 2 partials ⚠️
extensions/variants.go	57.14%	3 Missing ⚠️
types/parser/parse.go	91.66%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #253      +/-   ##
==========================================
+ Coverage   68.95%   69.02%   +0.07%     
==========================================
  Files          47       47              
  Lines       10935    10984      +49     
==========================================
+ Hits         7540     7582      +42     
- Misses       3034     3039       +5     
- Partials      361      363       +2     

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