Bound download expansion and escape control chars on the terminal (closes #164)

Objects fetched from the store are untrusted; several decode paths let one expand or print without limit.

- blobgen.LimitReader errors past a byte cap (not io.LimitReader silent EOF). DecodeManifest reads through caps on both compressed input and decompressed output, far above any real manifest, so json.Decode cannot buffer a compressible bomb. FetchAndDecryptBlob bounds decompression to the blob recorded uncompressed_size (not the restoring host blob_size_limit).
- downloadSnapshotDB streams straight from storage to its temp file with io.Copy, replacing two ReadAll calls that held the whole database twice.
- FetchBlob drops the per-blob Stat round-trip, its expectedSize parameter and returned size, all of which only fed a debug log.
- TTYHandler and ui.Writer escape control characters in messages, attribute keys/values, and rendered identifiers/paths before colour codes are applied, so a crafted value cannot drive the terminal.

Model: opus-4-8

internal/log/tty_escape_test.go

@@ -0,0 +1,39 @@
+package log_test
+
+import (
+	"bytes"
+	"log/slog"
+	"strings"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+	"sneak.berlin/go/vaultik/internal/log"
+)
+
+// TestTTYHandlerEscapesControlCharacters logs a message and an attribute
+// value that each carry an ESC and a newline — the shape a crafted path or
+// storage error from the destination would take — and checks neither raw
+// byte reaches the output. The handler's own colour codes (ESC ... m) are
+// stripped first; any ESC left after that came from the untrusted value.
+func TestTTYHandlerEscapesControlCharacters(t *testing.T) {
+	t.Parallel()
+
+	var buf bytes.Buffer
+
+	logger := slog.New(log.NewTTYHandler(&buf, debugHandlerOptions()))
+	logger.Info("start\x1b[31mZAP\nend", "target", "a\x1b[31mZAP\nb")
+
+	out := buf.String()
+
+	// The only newline is the line terminator; the injected ones were escaped.
+	require.Equal(t, 1, strings.Count(out, "\n"),
+		"a newline in the message or a value must be escaped, not emitted raw")
+
+	// After the handler's own colour codes are removed, no ESC survives.
+	stripped := ansiEscape.ReplaceAllString(out, "")
+	require.NotContains(t, stripped, "\x1b",
+		"a raw ESC from the message or a value must not reach the terminal")
+
+	// The escaped form is what appears instead.
+	require.Contains(t, out, `\x1b`)
+}