chain: pin that only an absent tip is a fresh chain

The fix in aa4c375 was correct and unpinned -- reverting it left the suite
green, which is the same failure this estate has now hit six times. Two tests
close it: an unreadable tip must surface its cause and must not report a first
run, and a tip of the wrong width must be refused as corruption rather than
rounded down to absence. Restoring the old condition now fails both.

The absent-tip half was already covered by TestOpenOnAnEmptyStoreStartsAtGenesisAndSaysSo,
so it is not restated here.

chain/open_test.go

@@ -0,0 +1,62 @@
+// Copyright (C) 2019-2026, Lux Industries Inc. All rights reserved.
+// See the file LICENSE for licensing terms.
+
+package chain
+
+import (
+	"errors"
+	"testing"
+
+	"github.com/luxfi/database"
+	"github.com/stretchr/testify/require"
+)
+
+// unreadableDB fails the tip read specifically, which is how a dead volume or a
+// closed handle presents: the key is there, the bytes will not come back.
+type unreadableDB struct {
+	database.Database
+	err   error
+	short bool
+}
+
+var errUnreadable = errors.New("volume gone")
+
+func (d *unreadableDB) Get(key []byte) ([]byte, error) {
+	if string(key) == string(tipKey) {
+		if d.short {
+			return []byte{1, 2, 3}, nil
+		}
+		return nil, d.err
+	}
+	return d.Database.Get(key)
+}
+
+// A chain that cannot read its tip does not know where it is. Reading that
+// failure as "nothing recorded yet" starts a live chain over at genesis and
+// lets it build height 1 on state it cannot see — durably, with the caller
+// told it succeeded. Only ABSENCE means a fresh chain.
+func TestOpenRefusesAnUnreadableTip(t *testing.T) {
+	f := newFixture(t)
+	require.NoError(t, f.store.Accept(newBlock(1, f.genesis.ID(), 1)))
+
+	// The same database, reopened behind a handle that cannot answer for the tip.
+	broken := New[*testBlock](&unreadableDB{Database: f.base, err: errUnreadable}, nil)
+
+	_, fresh, err := broken.Open(f.genesis, f.parse)
+	require.ErrorIs(t, err, errUnreadable, "the cause must survive the wrap")
+	require.False(t, fresh, "an unreadable tip is not a first run")
+}
+
+// A tip of the wrong width is corruption, not absence. Rounding it down to a
+// fresh chain would discard a live chain because one field was damaged.
+func TestOpenRefusesAWrongWidthTip(t *testing.T) {
+	f := newFixture(t)
+	require.NoError(t, f.store.Accept(newBlock(1, f.genesis.ID(), 1)))
+
+	broken := New[*testBlock](&unreadableDB{Database: f.base, short: true}, nil)
+
+	_, fresh, err := broken.Open(f.genesis, f.parse)
+	require.Error(t, err)
+	require.ErrorContains(t, err, "want")
+	require.False(t, fresh)
+}