docs(e2ee): investigation update — 41.7.0 delta + web-specific KE-1 root cause

Code-read + upstream-issue triage this session:
- 41.7.0 / crypto-wasm 18.3.1 does NOT fix KE-1 (no OTK/upload change; #5200
  still open) — the SDK-pin remediation lever is closed.
- Confirmed root cause = rust-crypto store <-> Synapse OTK divergence; the
  leading web trigger is that cinny never requests persistent storage, so the
  IndexedDB crypto store is evictable while the localStorage session survives.
- New buildable preventive mitigation: navigator.storage.persist() on login
  (+ multi-tab guard, 400-loop recovery prompt). Added as §6 with a secondary
  KE-2 to-device-validation hypothesis and capture discriminators.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

LOTUS_BUGS.md

@@ -112,6 +112,19 @@ signed_curve25519:AAAAAAAAAGQ already exists. Old key: {…} new key: {…}` —
   mismatch, OTK id-counter desync, RC-SDK (`41.6.0-rc.0`) regression, or a
   Synapse OTK bug. Repro signature: grep console for `already exists`.
   **Extreme — planning session.**
+  **Update 2026-07 (investigation §6):** upstream `matrix-rust-sdk#5200` (still
+  OPEN) confirms the mechanism — on the 400, `mark_request_as_sent()` never fires
+  so the SDK re-issues the identical upload forever. **`41.7.0` does NOT fix it**
+  (crypto-wasm 17→18.3.1 has no OTK/upload change; 18.3.x was to-device security
+  only) — the SDK-pin lever is closed. Root cause = **store↔server OTK
+  divergence**; the leading **web-specific** trigger is that cinny never calls
+  **`navigator.storage.persist()`**, so the IndexedDB crypto store is evictable
+  while the `localStorage` session/device-id survives → device resurrects with a
+  blank store → re-uploads OTKs the server still holds. **Actionable preventive
+  fix (buildable now, no call needed):** request persistent storage on login
+  (+ optional multi-tab guard + 400-loop→recovery-prompt). Healing an already-
+  diverged device still needs a clean **logout+login** (not just "clear
+  storage"). See `LOTUS_E2EE_INVESTIGATION.md` §6.
 
 - **KE-2 — Element Call media keys not arriving/decrypting → audio & video cut out (CRITICAL).**
   `MissingKey: missing key at index N for participant @user`, `skipping decryption