docs: record the measured steady-state nightly (1.24 TiB walked in 1m43s)

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

ks4/restic-backup.md

@@ -286,6 +286,12 @@ deleted.
 - **Done 2026-08-28**: initial sync (1,114,212 files / 1.239 TiB in
   4h10m, only 2.4 GiB added thanks to dedup + the seafile GC), restore
   drill passed (§7), interim ks2 push re-enabled.
+- **Steady state, measured 2026-08-29** (first full nightly run):
+  **1,114,343 files / 1.239 TiB walked in 1 min 43 s**, 201 MiB added
+  (63 MiB stored), `rc=0`. For comparison, the previous S3 tool needed
+  3–4 h per night for *one* of those trees. This is the local-cache
+  cost model working as intended: the walk is stat-only, and only the
+  churn is read, chunked and uploaded.
 - **Left**: the nuc leg after FTTH, then the ks2 decommission
   ([../ks2/plan.md](../ks2/plan.md)). The instance leg (§6) is
   shelved, not pending.