T11.6: retire the per-browser Usage report (CR-019)

Removes the old "Usage logs" card from admin.html/admin.js (the
usage-admin panel + loadUsage(), D5/T7.10) now that T11.5's real,
server-side Activity & usage card exists and reads real per-user data
instead of per-browser localStorage.

wp-usage.js is deleted outright, along with its three <script> includes
(admin.html, wp-creation-index.html, work-package-suite.html) - it had
no reader left once the panel above it was removed (the "download the
full event log" button lived only in that panel), and per the original
CR-019 decision record it was never reliably tied to a real identity,
so it was never a candidate data source for the new report either.

Its two call sites (wp-creation-app.js, work-package-suite-app.js) keep
a local track() function as a documented no-op rather than having each
of their ~45 individual track('event', ...) call sites deleted one at a
time - that would be a much larger, riskier diff for the same outcome
(no data is recorded either way), and each call site still marks what
was worth recording if usage analytics are ever rebuilt server-side.
work-package-suite-app.js's dwell-timer plumbing (_stepEnter /
trackStepDwell), which only ever fed track(), was left in place for the
same reason: inert, not broken.

Also removes tests/usage_check.py, which tested exactly the retired
feature, and updates its line in docs/reference/file-map.md to point at
the 2026-09-17 decision record instead.

Verified:
  - grep across the whole repo for WPUsage / wp-usage.js / usage-admin /
    usage_check: no live references remain, only explanatory comments
    and planning docs (decisions-2026-09-17.md, wave-11.md) that
    describe the removal itself
  - node --check on all three touched .js files: no syntax errors
  - full backend smoke test (27/27) and seed_demo.py still pass
  - tests/baseline_shots.py --pages admin,creator,sop at 390px/1440px,
    run locally: all three pages render with no new JS errors (the one
    "beforeunload" log line on sop/creator at 1440px is pre-existing
    harness noise from wp-autosave.js's unsaved-work guard, unrelated
    to this change) and no horizontal overflow; refreshed baseline
    screenshots committed alongside this change

docs/waves/wave-11.md

@@ -192,13 +192,37 @@ records before deciding — do not assume from this file alone).
 **Do not:** leave the old panel in place "just in case." Two activity reports
 showing two different numbers is worse than one.
 
+**Decision (2026-09-23):** `wp-usage.js` is removed, not kept — it had no
+reader left once the admin panel above it was removed (the "download the
+full event log" button lived only in that panel), and per the original
+decision record it was never reliably tied to a real identity, so it was
+never a candidate source for the new report either. The file itself and its
+three `<script>` includes (`admin.html`, `wp-creation-index.html`,
+`work-package-suite.html`) are gone. Its two call sites
+(`wp-creation-app.js`, `work-package-suite-app.js`) keep a local `track()`
+function as a documented no-op rather than having each of their ~45
+individual `track('event', …)` call sites deleted one at a time — that
+would be a far larger, riskier diff for the same outcome (no more data is
+recorded either way), and it keeps each call site as a marker of what was
+worth recording if usage analytics are ever rebuilt server-side. The
+dwell-timer plumbing that only ever fed `track()` (`work-package-suite-app.js`'s
+`_stepEnter`/`trackStepDwell`) was left in place for the same reason — it is
+inert now, not broken, and touching it buys nothing.
+
+Also removed: `tests/usage_check.py` (tested exactly the retired feature —
+D5/T7.10's per-browser analytics core and admin report) and its line in
+`docs/reference/file-map.md`, replaced with a note pointing at this decision.
+
 **Done when:**
 
-- [ ] the old `usage-admin` panel and its markup are gone from `admin.html`/
+- [x] the old `usage-admin` panel and its markup are gone from `admin.html`/
       `admin.js`
-- [ ] a decision on `wp-usage.js` itself is recorded (removed, or kept with a
-      stated reason) — not left ambiguous
-- [ ] nothing else in the app references the removed code; grep confirms
+- [x] a decision on `wp-usage.js` itself is recorded (removed, or kept with a
+      stated reason) — not left ambiguous — see above
+- [x] nothing else in the app references the removed code; grep confirms
+      (only remaining hits are this file, the 2026-09-17 decision record, and
+      the two explanatory code comments left at the retired call sites — all
+      prose, not live references)
 
 ---