docs(run-engine): fix stale references in the CK fairness design docs

Reframe the retained findings/research headers so they no longer say
'delete before merge' (the spike harness is archived and ships nothing; the
findings ship as a design reference), and replace the placeholder
.server-changes/2026-XX-XX filename with the real dated file. Addresses CodeRabbit
notes on the design docs.

internal-packages/run-engine/design/plans/2026-07-23-ck-virtual-time-scheduling-plan.md

@@ -236,7 +236,7 @@ Create:
   assertions)
 - `internal-packages/run-engine/src/run-queue/tests/ckVtimeConcurrency.test.ts`
   (multi-consumer correctness, op-count budget)
-- `.server-changes/2026-XX-XX-ck-fair-scheduling.md` (at PR time; note it
+- `.server-changes/2026-07-24-ck-fair-scheduling.md` (at PR time; note it
   ships dark)
 
 ## Tasks
@@ -752,7 +752,7 @@ Verify: `pnpm run typecheck --filter @internal/run-engine` and
 
 ### Task 10: ship notes and cleanup
 
-- Add `.server-changes/2026-XX-XX-ck-fair-scheduling.md` per
+- Add `.server-changes/2026-07-24-ck-fair-scheduling.md` per
   `.server-changes/README.md` (user-facing wording; it ships dark, so the
   note says the fair ordering exists behind a flag and changes nothing by
   default). No changeset (no public package touched).

internal-packages/run-engine/design/references/run-queue-fairness-base-queue-findings.md

@@ -1,7 +1,7 @@
 # Fair-queueing spike: findings
 
-This is a throwaway spike. It ships nothing and should be deleted before any
-merge to main; it exists to inform a design decision, not to become code.
+Findings from a throwaway spike whose harness is archived and ships nothing; these
+findings are retained here as a design reference that informed the change, not as code.
 
 Read the caveats section before quoting any single number. Two of them matter up
 front: (1) the candidate selectors are handed tenant identity (parsed from the

internal-packages/run-engine/design/references/run-queue-fairness-caps-vs-scheduling-findings.md

@@ -1,6 +1,6 @@
 # Caps vs scheduling: reconciliation findings
 
-Throwaway spike. Ships nothing; delete before any merge to main. Relative ranking
+Findings from a throwaway spike whose harness is archived (`chore/fair-queueing-spike`) and ships nothing; these findings are retained here as a design reference. Relative ranking
 on a small simulation, not a statistical or load study: read the verdicts as "what
 this harness supports", not proofs. Went through a blind two-model adversarial
 review (a third stalled); the review's fixes are folded in below.

internal-packages/run-engine/design/references/run-queue-fairness-ck-findings.md

@@ -1,6 +1,6 @@
 # Per-concurrency-key fairness spike: findings
 
-Throwaway spike. Ships nothing; delete before any merge to main.
+Findings from a throwaway spike whose harness is archived (`chore/fair-queueing-spike`) and ships nothing; these findings are retained here as a design reference.
 
 Bottom line: the base-queue spike's direction carries over to the real seam. At
 the concurrency-key grain the production baseline (serve the oldest-head CK

internal-packages/run-engine/design/references/run-queue-fairness-research.md

@@ -1,6 +1,6 @@
 # Queue-fairness research (grounding for the caps-vs-scheduling reconciliation)
 
-Throwaway spike notes. Five Fable research passes, distilled. Citations kept so
+Research notes distilled from a throwaway spike, retained here as a design reference. Five Fable research passes, distilled. Citations kept so
 the findings write-up and report can point at real sources. This grounds the
 central claim: occupancy caps and fair scheduling are orthogonal knobs, and the
 plan-of-record ships the cap knob while the earlier spike measured the scheduler