cindex "panic: no progress" in v1.3.0-rc1

In the changes between v1.2.0 and v1.3.0-rc1 (index version 2), cindex will now frequently panic with "no progress"

Here's a reproduction against the codesearch code itself, initializing with `cindex /home/test/codesearch` and then running `cindex` three times.

```
$ git clone https://github.com/google/codesearch
$ cd codesearch/cmd/cindex
$ git checkout -b bug v1.3.0-rc.1
$ go version
go version go1.25.5 X:nodwarf5 linux/amd64
$ go build
$ ./cindex -list
cindex: 2025/12/19 16:47:52 open /home/test/.csearchindex: no such file or directory
$ ./cindex /home/test/codesearch
cindex: 2025/12/19 16:47:58 index /home/test/codesearch
cindex: 2025/12/19 16:47:58 flush index
cindex: 2025/12/19 16:47:58 152741 data bytes, 152880 index bytes
cindex: 2025/12/19 16:47:58 done
$ ./cindex
cindex: 2025/12/19 16:48:00 index /home/test/codesearch
cindex: 2025/12/19 16:48:00 flush index
cindex: 2025/12/19 16:48:00 152741 data bytes, 152880 index bytes
cindex: 2025/12/19 16:48:00 merge /home/test/.csearchindex /home/test/.csearchindex~
cindex: 2025/12/19 16:48:00 done
$ ./cindex
cindex: 2025/12/19 16:48:02 index /home/test/codesearch
cindex: 2025/12/19 16:48:02 flush index
cindex: 2025/12/19 16:48:02 152741 data bytes, 152880 index bytes
cindex: 2025/12/19 16:48:02 merge /home/test/.csearchindex /home/test/.csearchindex~
cindex: 2025/12/19 16:48:02 done
$ ./cindex
cindex: 2025/12/19 16:48:02 index /home/test/codesearch
cindex: 2025/12/19 16:48:02 flush index
cindex: 2025/12/19 16:48:02 152741 data bytes, 152880 index bytes
cindex: 2025/12/19 16:48:02 merge /home/test/.csearchindex /home/test/.csearchindex~
panic: no progress

goroutine 1 [running]:
github.com/google/codesearch/index.Merge({0xc0000205e0, 0x1a}, {0xc000018048?, 0x5248f8?}, {0xc000020080, 0x19})
/home/test/codesearch/index/merge.go:201 +0x1005
main.main()
/home/test/codesearch/cmd/cindex/cindex.go:170 +0x8d7
```

Riproduci il problema compilando cmd/cindex a v1.3.0-rc.1 ed eseguendolo ripetutamente sul checkout di codesearch. Parti da index/merge.go:201 e segui la chiamata da cmd/cindex/cindex.go:170, confrontando il comportamento di merge ripetuto con l'output registrato. È fatto quando l'indicizzazione ripetuta viene completata senza il panic "no progress".

Scritto dal modello di indicizzazione a partire dal testo della issue.