2-byte drop in VNG file size

[philrz]

Repro is with Zed commit 88f40ed and the test data file sample.tsv from the Zui repo.

At commit c29dd97 right before the changes in #4812, output the sorted file as VNG.

$ zq -version
Version: v1.10.0-14-gc29dd976

$ zq -f vng 'sort ts' sample.tsv > /tmp/c29dd97.vng

Repeat the same at commit 88f40ed, which is associated with the changes in #4812.

$ zq -version
Version: v1.10.0-15-g88f40edd

$ zq -f vng 'sort ts' sample.tsv > /tmp/88f40ed.vng

We find the newer VNG output is 2 bytes smaller than before.

$ ls -l /tmp/*.vng
-rw-r--r--  1 phil  wheel  7740 Oct 24 10:35 /tmp/88f40ed.vng
-rw-r--r--  1 phil  wheel  7742 Oct 24 10:34 /tmp/c29dd97.vng

Since the changes in #4812 impacted the amount of memory used to store values, I thought this might have been an expected result, but @nwt confirmed it's unexpected. Hence this issue.

[philrz]

@nwt took a closer look and confirmed this is expected after all. Will plan to update the related Zui test to expect this change. Closing.