docs(headless): record K4 telemetry checkpoint

docs/plans/2026-07-26-modern-runtime-slice-k.md

@@ -341,6 +341,18 @@ disposes only after the final host acknowledgement.
 
 ### K4 — 30-session stress and closeout
 
+K4 telemetry checkpoint `cb512fd0` is complete. One process-level
+absolute-deadline sampler now reports CPU, private/resident/managed memory,
+GC, threads, handles, Linux descriptors/sockets, session/ownership debt,
+shared-content leases, scheduler wakeups, and deadline lateness without adding
+per-session workers or timers. The first native Linux two-account run exposed
+and corrected both pre-connect deadline arming and a sub-millisecond
+`Task.Delay` busy loop. Windows and native Ubuntu Headless tests pass 56/56;
+the complete Release suite passes 8,773 tests / 5 skips. The user requested
+that the active two-client observer process remain running, so the exact fixed-
+binary connected baseline and numeric ceilings remain next. Evidence:
+`docs/research/2026-07-27-slice-k4-resource-telemetry-checkpoint.md`.
+
 - Measure a 1-session warm baseline, then 5, 10, and 30 sessions.
 - Before the final run, commit numeric ceilings derived from K3 evidence for:
   - incremental private bytes per established session;