feat: virtual PyPI repos can merge local + remote members (#51)

## Summary
- Virtual engine detects local members and generates indexes in-memory
- MemberIndex.RepoType drives correct URL prefix in merged output
- PyPI merger rewrites links to /api/v1/local/ or /api/v1/remote/ appropriately
- Includes local PyPI support (cherry-picked from #50)

## Test plan
- [x] Upload wheel to local PyPI → install from direct local URL
- [x] Create virtual with local + remote → install from virtual URL
- [x] Both paths produce correct absolute download URLs

Reviewed-on: #51
Co-authored-by: Ben Vincent <ben@unkin.net>
Co-committed-by: Ben Vincent <ben@unkin.net>

internal/virtual/engine.go

@@ -5,6 +5,7 @@ import (
 	"fmt"
 	"io"
 	"log/slog"
+	"strings"
 	"sync"
 
 	"git.unkin.net/unkin/artifactapi/internal/database"
@@ -13,13 +14,18 @@ import (
 	"git.unkin.net/unkin/artifactapi/pkg/models"
 )
 
+type LocalIndexGenerator interface {
+	GeneratePyPIPackageHTML(ctx context.Context, repoName, packageName string) ([]byte, error)
+}
+
 type Engine struct {
 	db          *database.DB
 	proxyEngine *proxy.Engine
+	localGen    LocalIndexGenerator
 }
 
-func NewEngine(db *database.DB, proxyEngine *proxy.Engine) *Engine {
-	return &Engine{db: db, proxyEngine: proxyEngine}
+func NewEngine(db *database.DB, proxyEngine *proxy.Engine, localGen LocalIndexGenerator) *Engine {
+	return &Engine{db: db, proxyEngine: proxyEngine, localGen: localGen}
 }
 
 func (e *Engine) Fetch(ctx context.Context, virt models.Virtual, path string, proxyBaseURL string) ([]byte, string, error) {
@@ -73,6 +79,16 @@ func (e *Engine) fetchMemberIndexes(ctx context.Context, virt models.Virtual, pa
 				return
 			}
 
+			if remote.RepoType == models.RepoTypeLocal {
+				body, err := e.fetchLocalIndex(ctx, *remote, virt.PackageType, path)
+				if err != nil {
+					results[idx] = result{err: fmt.Errorf("local index %q: %w", name, err)}
+					return
+				}
+				results[idx] = result{index: MemberIndex{RemoteName: name, RepoType: remote.RepoType, Body: body}}
+				return
+			}
+
 			prov, err := provider.Get(remote.PackageType)
 			if err != nil {
 				results[idx] = result{err: fmt.Errorf("provider %q: %w", remote.PackageType, err)}
@@ -92,7 +108,7 @@ func (e *Engine) fetchMemberIndexes(ctx context.Context, virt models.Virtual, pa
 				return
 			}
 
-			results[idx] = result{index: MemberIndex{RemoteName: name, Body: body}}
+			results[idx] = result{index: MemberIndex{RemoteName: name, RepoType: remote.RepoType, Body: body}}
 		}(i, memberName)
 	}
 
@@ -109,3 +125,20 @@ func (e *Engine) fetchMemberIndexes(ctx context.Context, virt models.Virtual, pa
 
 	return members, nil
 }
+
+func (e *Engine) fetchLocalIndex(ctx context.Context, remote models.Remote, packageType models.PackageType, path string) ([]byte, error) {
+	switch packageType {
+	case models.PackagePyPI:
+		if e.localGen == nil {
+			return nil, fmt.Errorf("no local index generator configured")
+		}
+		parts := strings.SplitN(strings.TrimPrefix(path, "simple/"), "/", 2)
+		pkgName := strings.TrimSuffix(parts[0], "/")
+		if pkgName == "" {
+			return nil, fmt.Errorf("cannot determine package name from path %q", path)
+		}
+		return e.localGen.GeneratePyPIPackageHTML(ctx, remote.Name, pkgName)
+	default:
+		return nil, fmt.Errorf("local index generation not supported for %q", packageType)
+	}
+}