docs(adr): ADR 009 requires a zero-sign property test for sums

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VdfEGXA7aJ6ftMsCc9PJze

context/adr/009-incremental-evaluation.md

@@ -439,6 +439,8 @@ Step 8's required tests:
   frame stay published;
 - a dropped connection or a failed request triggers a correction, and a later request
   with the same session resumes from the watermark.
+- a zero sum has the sign IEEE gives the same addends: `-0.0` only if every addend is
+  `-0.0`, and `+0.0` for an exact cancellation (a property test against IEEE).
 
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