Reject a metadata database truncated to the age header and nonce (closes #152)

An object holding exactly the age header plus its 16-byte nonce decrypts
without error: age.Decrypt succeeds, and on the first read the age stream's
io.ErrUnexpectedEOF is mapped by the zstd decoder to a clean io.EOF at frame
start. blobgen.Reader.Read then reported zero bytes and no error, so a
truncated stream was indistinguishable from a valid empty one. Blobs are
caught by the Close-time hash check, but the metadata database export was not:
restore built a fresh schema on the empty file and exited reporting success.

blobgen.Reader.Read now, on EOF, reads once more from the age reader and
surfaces io.ErrUnexpectedEOF unless that read is (0, io.EOF) — the state a
genuine end leaves behind, so an empty payload still round-trips to empty with
no error. downloadSnapshotDB additionally rejects a zero-length decrypted
database before OpenReadOnly applies a schema.

On next the decrypted-DB identity check already rejects an empty database, so
the restore-level test guards the required end-to-end property; the blobgen
tests are the ones that fail without the reader change.

Model: opus-4-8

internal/blobgen/reader.go

@@ -2,6 +2,7 @@ package blobgen
 
 import (
 	"crypto/sha256"
+	"errors"
 	"fmt"
 	"hash"
 	"io"
@@ -56,6 +57,22 @@ func (r *Reader) Read(p []byte) (int, error) {
 	n, err := r.teeReader.Read(p)
 	r.bytesRead += int64(n)
 
+	// When the ciphertext is cut right after the age header plus its
+	// 16-byte nonce, the age reader's first read fails with
+	// io.ErrUnexpectedEOF, and the zstd decoder maps that to a clean
+	// io.EOF at frame start. That makes a truncated stream look like a
+	// valid empty one. Distinguish the two: on EOF, read once more from
+	// the age reader. A genuine end leaves it at (0, io.EOF); a truncated
+	// stream leaves its stored io.ErrUnexpectedEOF, which we surface.
+	if errors.Is(err, io.EOF) {
+		var probe [1]byte
+
+		m, ageErr := r.decryptor.Read(probe[:])
+		if m != 0 || !errors.Is(ageErr, io.EOF) {
+			return n, io.ErrUnexpectedEOF
+		}
+	}
+
 	return n, err
 }