certificate chain walker + progress bar fix

- aggregator.CertChain: walks previous_hash from head until genesis_signature
- cmd: 'cert' subcommand, -chain flag for full walk, 'head' shortcut resolves
  latest snapshot's certificate_hash
- ProgressFn now signals both bytes-read and total-from-Content-Length so
  percent is computed against the actual transfer size, not the uncompressed
  target
- verified against preprod: 90-cert chain head→genesis, Ed25519 genesis cert
  shape (64-byte sig over 32-byte signed_message, protocol_message carries
  next_aggregate_verification_key for BLS), STM-signed non-genesis certs

pipeline is now verification-sprint ready

internal/aggregator/client.go

@@ -158,3 +158,32 @@ func (c *Client) GetCertificate(ctx context.Context, hash string) (*Certificate,
 	}
 	return &out, nil
 }
+
+// CertChain walks previous_hash backwards from headHash until it hits the
+// first certificate that carries a genesis_signature. Returns the chain
+// ordered head-first. Caller can invert if root-first is preferred.
+//
+// The chain length is usually 1-3 certs per epoch boundary; an unbounded
+// walk would be a footgun so it caps at maxDepth.
+func (c *Client) CertChain(ctx context.Context, headHash string, maxDepth int) ([]*Certificate, error) {
+	if maxDepth <= 0 {
+		maxDepth = 1024
+	}
+	var chain []*Certificate
+	next := headHash
+	for i := 0; i < maxDepth; i++ {
+		if next == "" {
+			return nil, fmt.Errorf("chain broke at depth %d: no previous_hash", i)
+		}
+		cert, err := c.GetCertificate(ctx, next)
+		if err != nil {
+			return nil, fmt.Errorf("depth %d (%s): %w", i, next, err)
+		}
+		chain = append(chain, cert)
+		if cert.GenesisSignature != "" {
+			return chain, nil
+		}
+		next = cert.PreviousHash
+	}
+	return nil, fmt.Errorf("cert chain exceeded max depth %d without reaching genesis", maxDepth)
+}

internal/artifact/download.go

@@ -14,6 +14,10 @@ import (
 	"time"
 )
 
+// ProgressFn is called with (bytesRead, totalBytes) where totalBytes is
+// taken from the response Content-Length (0 if unknown — server didn't send).
+type ProgressFn func(read, total int64)
+
 // Download fetches a URL to destPath, resuming from a .part file if one
 // exists. If expectedSHA256 is non-empty, the final file is integrity-checked.
 // Progress is reported via the supplied callback (called with current bytes).
@@ -25,7 +29,7 @@ import (
 //   - Resume is implemented via the HTTP Range header against the existing
 //     .part file size; falls back to full download if the server refuses.
 //   - destPath is atomically replaced only after SHA validation passes.
-func Download(ctx context.Context, uri, destPath, expectedSHA256 string, progress func(bytes int64)) error {
+func Download(ctx context.Context, uri, destPath, expectedSHA256 string, progress ProgressFn) error {
 	if err := os.MkdirAll(filepath.Dir(destPath), 0o755); err != nil {
 		return fmt.Errorf("mkdir: %w", err)
 	}
@@ -77,8 +81,15 @@ func Download(ctx context.Context, uri, destPath, expectedSHA256 string, progres
 		}
 	}
 
+	// totalBytes = existing + response.ContentLength if server sent one;
+	// for resumed partial responses Content-Length is the remaining, not total.
+	var totalSize int64
+	if resp.ContentLength > 0 {
+		totalSize = existing + resp.ContentLength
+	}
+
 	w := io.MultiWriter(out, h)
-	total := existing
+	read := existing
 	buf := make([]byte, 256*1024)
 	lastProgress := time.Now()
 	for {
@@ -87,9 +98,9 @@ func Download(ctx context.Context, uri, destPath, expectedSHA256 string, progres
 			if _, werr := w.Write(buf[:n]); werr != nil {
 				return fmt.Errorf("write: %w", werr)
 			}
-			total += int64(n)
+			read += int64(n)
 			if progress != nil && time.Since(lastProgress) > 250*time.Millisecond {
-				progress(total)
+				progress(read, totalSize)
 				lastProgress = time.Now()
 			}
 		}
@@ -101,7 +112,7 @@ func Download(ctx context.Context, uri, destPath, expectedSHA256 string, progres
 		}
 	}
 	if progress != nil {
-		progress(total)
+		progress(read, totalSize)
 	}
 	if err := out.Close(); err != nil {
 		return err
@@ -120,7 +131,7 @@ func Download(ctx context.Context, uri, destPath, expectedSHA256 string, progres
 var ErrNoLocations = errors.New("no download locations available")
 
 // DownloadFirst tries each URI in order until one succeeds.
-func DownloadFirst(ctx context.Context, uris []string, destPath, expectedSHA256 string, progress func(int64)) error {
+func DownloadFirst(ctx context.Context, uris []string, destPath, expectedSHA256 string, progress ProgressFn) error {
 	if len(uris) == 0 {
 		return ErrNoLocations
 	}