Merge RB-29 — resolve the body datum placeholder from at, not UtcNow

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

libs/shared/docs/behaviour-spec.mdx

@@ -21,7 +21,7 @@ tested where._
 Every bullet below is a real test name from the suite — an `it()` title (frontend) or a test
 method name (backend), read as a sentence. Nothing here is hand-written prose: this page
 **is** the suite, reshaped for a business reader. 467 frontend behaviours across
-9 contexts; 238 backend behaviours across 41 test
+9 contexts; 240 backend behaviours across 41 test
 classes.
 
 ## Frontend (by context)
@@ -1102,6 +1102,8 @@ classes.
 ### LetterHtmlTests
 
 - Render matches the golden file
+- Render resolves the body datum placeholder from the given at not the wall clock
+- Render keeps the letterhead date and the body datum in agreement for a historical at
 - Every letter prefixed class exists in letter css
 
 ### NotificatieTests