perf(codegen): replace source importer with gcexportdata for ~1000x faster type resolution (MadAppGang#58)

* perf(codegen): replace source importer with gcexportdata for ~1000x faster type resolution

The type resolver previously used importer.ForCompiler("source") which
recursively parses all transitive dependencies from .go source files.
For large dependency graphs (e.g., Cosmos SDK), this took 15+ minutes.

Now uses gcexportdata to read pre-compiled type data from Go's build
cache via `go list -export -deps`. Benchmarks show:
- go list -export -deps: ~42ms (finds all dependency export files)
- gcexportdata.Read() for all deps: ~23ms
- Cached (second call, same process): 750ns

The gcExportImporter also caches `go list` results across multiple
NewTypeResolver calls, eliminating the duplicate invocation between
Phase 0 (semantic validation) and Step 2.1 (error propagation).

Falls back to source importer if `go list -export` fails.

Addresses MadAppGang#38

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* fix(codegen): remove GOFLAGS clearing and capture go list stderr

Review feedback from 6-model team review:
- Remove GOFLAGS= override that broke vendor mode projects
- Capture go list stderr for better error diagnostics
- File descriptor leak was a false positive (os.Open error IS checked
  before defer f.Close() is registered)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: erudenko

pkg/codegen/gc_export_importer.go

@@ -106,11 +106,13 @@ func NewGCExportImporter(fset *token.FileSet, workingDir string, patterns ...str
 
 	cmd := exec.Command("go", args...)
 	cmd.Dir = workingDir
-	cmd.Env = append(os.Environ(), "GOFLAGS=") // Clear GOFLAGS to avoid interference
+
+	var stderr bytes.Buffer
+	cmd.Stderr = &stderr
 
 	output, err := cmd.Output()
 	if err != nil {
-		return nil, fmt.Errorf("gcExportImporter: go list failed: %w", err)
+		return nil, fmt.Errorf("gcExportImporter: go list failed: %w (stderr: %s)", err, stderr.String())
 	}
 
 	// Parse the streaming JSON output (one object per package, NOT a JSON array)