feat: add support for cross-package generic types

[danielthomas74]

Description

Add cross-package generic type support to the code generator. Generic types referenced across package boundaries (e.g. private.Envelope[server.Item]) now carry their type arguments through parsing, resolution, and into both Go and TypeScript code generation.

Type of Change

• 🐛 Bug fix
• ✨ New feature
• 💥 Breaking change
• 📝 Documentation
• ♻️ Refactoring
• ⚡ Performance
• ✅ Tests
• 🔧 Build/CI

Changes

• model.Value gains TypeArgs []*Value slot; canonical home for generic instantiations regardless of underlying Scalar/StructType kind.
• internal/parser/typereader.go: IndexExpr / IndexListExpr push directly to v.TypeArgs after recursing into the generic base type. Handles both cross-package selector idents (parsed as Scalar) and same-package idents (parsed as StructType).
• internal/parser/servicereader.go: loadFlatStructsValue, needsWorkValue, getStructTypesForField, getScalarForField walk value.TypeArgs uniformly so dependency resolution and import collection reach into generic arguments.
• internal/codegen/gocode.go: new renderGoTypeArgs helper; both Scalar and StructType render paths emit [A, B]. extractImportValue walks TypeArgs so cross-package args are imported.
• internal/codegen/typescript.go: renderTSTypeArgs called from Scalar and StructType branches so cross-package generics render <A, B> correctly.
• tests/generics/: new test fixture covering same-package + cross-package generics, pointer-to-generic, nested generics, generics of foreign types. Bun-based TS client tests roundtrip through a real server.

Checklist

• My code adheres to the coding and style guidelines of the project.
• I have performed a self-review of my own code.
• I have commented on my code, particularly in hard-to-understand areas.
• I have made corresponding changes to the documentation.