feat(go): Add Go stdlib registry data structures and types (PR-01) - #546

Summary

Introduce all data structures and type definitions required for Go standard library registry support (PR-01 of the Go stdlib support series).

• Add go_stdlib_types.go with 13 typed structs covering the full Go stdlib registry schema: GoStdlibRegistry, GoManifest, GoVersionInfo, GoPackageEntry, GoRegistryStats, GoStdlibPackage, GoStdlibFunction, GoFunctionParam, GoReturnValue, GoTypeParam, GoStdlibType, GoStructField, GoStdlibConstant, GoStdlibVariable
• Add GoCallEdge struct to types.go with IsStdlib bool and Confidence float32 fields for call resolution metadata
• Add GoStdlibLoader interface to types.go to decouple core from the registry package (avoids import cycles)
• Add StdlibLoader GoStdlibLoader field to GoModuleRegistry for lazy stdlib metadata attachment
• All JSON tags use snake_case matching the Go stdlib CDN registry format
• All structs include constructor functions and helper methods (GetPackage, GetFunction, GetType, GetConstant, GetVariable, HasPackage, PackageCount, etc.)

Test plan

• go test ./graph/callgraph/core/... — 36 new tests, all passing
• golangci-lint run ./graph/callgraph/core/... — 0 issues
• go build ./... — builds cleanly
• 100% coverage on all new code in go_stdlib_types.go and new additions to types.go
• JSON round-trip tests verify snake_case tag names for every multi-word field
• Table-driven tests cover all nil/missing/found paths for every lookup method
• Integration test validates end-to-end registry workflow

Dependencies

Prerequisite for:

• PR-21: Extraction tool (tools/generate_go_stdlib_registry.go)
• PR-22: Remote loader (registry/go_stdlib_remote.go)
• PR-23: R2 publishing pipeline

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