#561 fix: --dump-logits must not promise rows it will never write

描述

## What

`--dump-logits` promised rows it never wrote.

A native pack leaves the per-token loop at

    if (native_pack) { spec_pos = pos; break; }

before the site that dumps a row per position, but the header was written at open. The file claimed
64 or 66 rows and contained **none** - and a reader that trusted it reported **"0 differing
positions"** from a file with no positions in it, which is the one answer that looks like a pass.

The header is now written **with the first row**. A run that dumps nothing leaves an **empty file**,
which is what actually happened, and a reader can treat size zero as "this mode never dumps" rather
than as agreement.

## Verified

On the native pack the file is now 0 bytes where it was 8. The non-native path is unchanged apart from
*when* the header lands, and was not exercised here - no non-native model is set up on the machine
this was measured on.

## Split out of

[#463](https://github.com/Niko1221/Strata/pull/463), which had grown two unrelated fixes. This one is
self-contained and independent of it.