docs(603): correct the record's own no-backfill claim, which the backfill commit falsifies

Caught in re-review. The record shipped in 0ad02db6 said 'no backfill was done
here'; 8e1e15e4 then backfilled three ci.* records on the same branch. Corrected
now while the record is still NEW at HEAD, so the body-diff guard does not apply
and no edit token is needed -- after merge this would be surviving rationale
prose and the same fix would cost one.

Deliberately avoiding the literal bracketed token string here: decisions_validate
matches it as a bare substring over the whole commit range, so a message merely
DESCRIBING the mechanism arms it and suppresses the rationale-rewrite guard for
the entire PR. Found exactly that way on the first draft of this commit.

docs/decisions.md

@@ -3761,9 +3761,11 @@ second ingestion format alongside MemPalace for the same corpus.
 **What it does have that we lacked is lifecycle and provenance metadata.** Records that encode
 measured facts about hosts, runners or upstream behavior drift silently — nothing marked them due for
 re-confirmation. `stale-after` fixes that, and `Sources:` gives the *evidence* a record rests on a
-home other than buried prose. Both are additive: no format migration, nothing changes for the
-existing corpus, and no backfill was done here — adopt them opportunistically on records that
-actually assert an external fact.
+home other than buried prose. Both are additive: no format migration, and nothing changes for a
+record that adopts neither. Adopt them opportunistically, on records that actually assert an external
+fact — the three seeded here (`ci.runner-placement`, `ci.infra-shaped-red-under-load`,
+`ci.peak-anon-measurement`) are the ones encoding measured host behavior, with dates staggered so
+they don't all come due in the same week.
 
 **The staleness check is non-blocking, and the generated catalog is clock-free — both on purpose.**
 A calendar date turning an unrelated PR red, or silently drifting `docs/decisions/README.md` into a