`PageBuilder` keeps 256 bytes back for the envelope it still has to write, so a
field appended after `Close()` is spent outside that reserve. It fits today by a
wide margin — the largest measured page was 524,086 of a 524,288 budget, and the
budget is half the line cap on purpose — but "nothing is written after Close()"
is the invariant worth having, because the next family to page will copy this.
No behaviour change; the field moves earlier in the same object.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016wDDVXWMDz82WqE1i969r4
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