e2e: 5 pre-existing failures on main (insert-modes append/prepend/sentinel, animation stagger, pagination last-page)

[ErickXavier]

Found while gating WS1 of PERF-PLAN (perf/ws1-template-plan). These tests fail with a build of current main (cdc56f0) in a fresh local run, so they are not caused by the perf branch stack:

Test	Browsers
insert-modes 1 — get-insert="append"	chromium, firefox, webkit
insert-modes 2 — get-insert="prepend"	chromium, firefox, webkit
insert-modes 4 — sentinel element exists in append mode	chromium, firefox, webkit
animations 3 — Stagger: incremental animation delays	chromium, firefox, webkit
pagination 4 — end-of-data via X-NoJS-Last-Page header	chromium (intermittent)

Evidence:

• Bisection: same failures with builds of main, feat/reactive-perf, and perf/ws1-template-plan — while plugin-system 2 (a real branch regression, fixed in de741ca) flipped between builds. So these do not depend on the perf changes.
• Not port-related: reproduced on E2E_PORT=3999 and 4123.
• insert-modes symptom: getByTestId('item') never renders on /e2e/examples/pagination-button.html — the mocked **/api/items* content never gets inserted (element(s) not found after 5 s).

Possibly environment drift (Playwright/browser versions) or a genuine get/get-insert regression that predates the perf work. Needs its own investigation on a fix/ branch off main.

Full logs from the gating session available on request.