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

html/wp-creation-index.html

@@ -11,7 +11,6 @@
 <!-- Addressable state (S3). Parses before the app scripts, which read the URL
      during their own boot. -->
 <script src="wp-url.js"></script>
-<script src="wp-usage.js"></script>
 <!-- Autosave, unsaved-work guard, draft recovery (S2). -->
 <script src="wp-autosave.js"></script>
 <!-- Which sections this project uses (CR-006). The same file the SOP wizard