TE-007: Render already accepts the letter's instant and uses it correctly for
the letterhead, but the body's datum placeholder resolved through ResolveAuto,
which ignored at and read DateTimeOffset.UtcNow. Threaded at through
RenderParagraphs and RenderNode, both already in Render's call chain with at
in scope. Zero public API change, zero call-site change. The bug this
prevents: re-rendering an archive or back-dating a letter would otherwise make
the letterhead and body dates disagree within a single document.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
# Conflicts:
# docs/project/refactor-backlog-setup/refactor-backlog/99-backlog.md
eho
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