Extract elapsedAtLeast to core/util, dedupe the throttle rationale, and correct the alloc-count/PLAN citations

docs/PLAN.md

@@ -850,20 +850,24 @@ the projects where the switch is slowest.
 `enumerateLiveGuids` and their cadence inside `bankPanelRefresh`). **Does not own:** any other
 panel TU, `view.cpp`, or the auto-tag RULES — only how often they are asked.
 
-**Behavior.** `detectNewContent` (`panel_input.cpp:146`) runs `enumerateLiveGuids` (`:106-135`)
-every tick of REAPER's `"timer"` registration (`main.cpp:341`, ~30/sec), regardless of panel
-state — O(T + I) REAPER string calls plus ~4(T+I) allocations per tick, on every project. Throttle
-the ENUMERATION, not the detection semantics: the same GUIDs must still be detected, the
-first-poll and `reloadPending` re-baseline guards must still fire before any diff, and the
-lane-minting pass must still run only on a tick that tagged. A slower cadence is a latency
-change to an invisible background tag, and the track states the chosen interval and why.
+**Behavior.** `detectNewContent` (`panel_input.cpp:147`) runs `enumerateLiveGuids` (`:106-135`)
+every tick of REAPER's `"timer"` registration (`main.cpp:354`, ~30/sec), regardless of panel
+state — O(T + I) REAPER string calls plus roughly 5 allocations per track and 6 per item per
+tick (a per-element-type count, not the earlier `~4(T+I)` derivation, which undercounted; the
+ratio the throttle buys is unaffected by the correction). Throttle the ENUMERATION, not the
+detection semantics: the same GUIDs must still be detected, the first-poll and `reloadPending`
+re-baseline guards must still fire before any diff, and the lane-minting pass must still run
+only on a tick that tagged. A slower cadence is a latency change to an invisible background
+tag, and the track states the chosen interval and why.
 
 **Acceptance criteria.**
 - New tracks and items are still auto-tagged into the active mode, and a project reload still
   re-baselines rather than mass-tagging.
 - The Ρ-W1-T1 rule survives: an added GUID that already carries a membership record is still
   dropped, so a render-in-place result is not re-tagged to Design.
-- The per-tick allocation count is measured before and after.
+- The per-tick allocation count is derived, not measured: it needs live REAPER-side track/item
+  counts and a runtime allocation trap neither CTest nor this repo's tooling provides.
+  `[verify — DAW]`.
 - The panel's own repaint cadence is unaffected.
 
 **Prerequisites.** None beyond Ω-W1. **Discharges:** Ω.7's polling cost.