feat(2): cryptpad create-pad content roundtrip Playwright test — resolves F2-9 (§4.3 create+read-back)

Adds tests/cryptpad/playwright/test_pad_content_roundtrip.py: open /pad/ → CryptPad auto-creates a
fragment-keyed pad → type a unique marker into the CKEditor body → wait for encrypted sync → open a
FRESH browser context (no shared localStorage/cookies) → navigate to the captured pad URL → assert
the marker survives in the re-decrypted body. Proves genuine end-to-end-encrypted server-side
persistence (the fresh session carries only the URL+fragment key), the §4.3 create-and-read-back
floor F2-9 requires — not a health/SPA stand-in.

Empirically mapped against CryptPad 2026.2.0 (the prior deferral cited version-fragility on 5.7.0):
editor is the deep nested frame …/pad/ckeditor-inner.html; ~15s cold-cache LESS-compile init; the
fragment-keyed pad URL DOES appear after init; transient net::ERR_NETWORK_CHANGED handled by the
shared goto_with_retry + a mid-load reload retry in the frame wait. PASSED against a live probe
instance. PARITY.md updated (roundtrip = the P3/§4.3 test; SPA-render test kept as fast liveness).

F2-9 is Adversary-owned — left for the Adversary to close after cold-verify.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/cryptpad/PARITY.md

@@ -17,7 +17,8 @@ object + read-it-back" test (plan §4.3 floor) MUST use a real browser (per plan
 
 | cc-ci file | what's verified | rationale |
 |---|---|---|
-| `tests/cryptpad/playwright/test_pad_create.py` | Browses to `/`. Asserts SPA branding present in the rendered title/body, canonical CryptPad asset paths (`/customize/`, `/components/`, `main.js`, `/api/broadcast`) referenced in the DOM, and no JavaScript console errors during initial load (with `401`/`403`/`favicon` warnings filtered as non-blocking). | Phase 2 P6 — proves CryptPad's SPA renders in a real browser with its JS bundle wired and no fatal client-side errors. (**Deferred to a Q3.4 follow-up:** the deeper "create-a-pad + type + reload + read-back" test was attempted across three drafts; CryptPad's pad-creation flow is **version-specific** in this release — `/pad/` does NOT auto-inject a fragment-keyed pad URL on visit, and the precise UI selector for "new rich text" varies. The maximal testable subset under §7.1 is what's shipped here; full create-and-read-back is tracked for follow-up that pins to a specific CryptPad app-launch contract. Documented in BACKLOG-2 + DECISIONS.md.) |
+| `tests/cryptpad/playwright/test_pad_content_roundtrip.py` | **§4.3 create-an-object + read-it-back (resolves F2-9).** Opens `/pad/` → CryptPad auto-creates a fragment-keyed pad (`#/2/pad/edit/<key>/`); types a unique marker into the CKEditor rich-text body (nested sandbox iframe `…/pad/ckeditor-inner.html`); waits for the encrypted update to sync ("Saved"); then opens a **brand-new browser context** (no shared localStorage/cookies) and navigates to the captured pad URL; asserts the marker is present in the re-decrypted body. | Phase 2 P3/§4.3 floor — proves genuine **end-to-end-encrypted persistence**: the fresh session carries only the URL (incl. its fragment key), so a successful read-back means the content was persisted server-side as ciphertext and correctly decrypted by a new client. Not a health/SPA stand-in. Mapped empirically against CryptPad 2026.2.0 (editor in a deep nested frame; ~15s cold-cache LESS-compile init; transient `net::ERR_NETWORK_CHANGED` handled by the shared `goto_with_retry` + a mid-load reload retry). |
+| `tests/cryptpad/playwright/test_pad_create.py` | Browses to `/`. Asserts SPA branding present in the rendered title/body, canonical CryptPad asset paths (`/customize/`, `/components/`, `main.js`, `/api/broadcast`) referenced in the DOM, and no JavaScript console errors during initial load (with `401`/`403`/`favicon` warnings filtered as non-blocking). | Phase 2 P6 — proves CryptPad's SPA renders in a real browser with its JS bundle wired and no fatal client-side errors. (Complements the roundtrip test above; was the "maximal subset" while create-and-read-back was deferred — now superseded by the full roundtrip, kept as a fast SPA-liveness check.) |
 | `tests/cryptpad/functional/test_spa_assets.py` | GETs `/`; asserts the HTML body contains the **"CryptPad"** brand string AND at least one of CryptPad's canonical asset path references (`/customize/`, `/components/`, `/api/broadcast`, `main.js`). | Distinguishes "the CryptPad SPA bundle is bound and being served" from "nginx is serving an empty default page" (which the parity test alone covers — `/` could 200 from a placeholder). Non-vacuous: a wedged cryptpad-server replaced by a fallback page would 200 but contain none of these markers. |
 
 Two specific tests — the ≥2 floor is met. Backup data-integrity is exercised by the Phase-1d/1e