fix(deb): normalize apt flat-repo ./ dist prefix in local index

Real apt appends the flat-repo dist "./" verbatim, requesting <repo>/./Packages
(and ./Release, ./InRelease). ServeLocalIndex matched the literal filename and
404d, breaking the documented `deb ... <repo>/ ./` one-liner; curl/browsers
pre-normalize /./ which masked it. Collapse the dot-segment via a small
normalizeIndexPath helper before matching, a no-op for pool/*.deb lookups.

internal/provider/deb/deb.go

@@ -14,6 +14,7 @@ import (
 	"io"
 	"log/slog"
 	"net/http"
+	"path"
 	"regexp"
 	"strconv"
 	"strings"
@@ -305,7 +306,16 @@ func parseControlFields(control string) map[string]string {
 	return fields
 }
 
-func (p *Provider) ServeLocalIndex(w http.ResponseWriter, r *http.Request, files provider.FileStore, repoName, path string) bool {
+// normalizeIndexPath collapses apt's verbatim dist prefix from a flat-repo
+// request. For `deb ... <repo>/ ./`, apt appends the "./" dist literally and asks
+// for "./Packages" (and "./Release", "./InRelease"); dot-segments must be
+// collapsed so the index matcher sees "Packages". A no-op for pool/*.deb paths.
+func normalizeIndexPath(p string) string {
+	return strings.TrimPrefix(path.Clean("/"+p), "/")
+}
+
+func (p *Provider) ServeLocalIndex(w http.ResponseWriter, r *http.Request, files provider.FileStore, repoName, reqPath string) bool {
+	path := normalizeIndexPath(reqPath)
 	switch path {
 	case "Packages", "Packages.gz", "Release":
 	default: