docs: add Element Call fork handoff + tag all EC-FORK references

Captures the plan to fork element-hq/element-call and build it from source for
true ownership of the in-call experience (decorations, focus/screenshare,
reconnect mic, native theming, call-audio injection) — none of which are fixable
against the prebuilt @element-hq/element-call-embedded bundle we ship today.

- New HANDOFF_ELEMENT_CALL_FORK.md: self-contained plan for a fresh session
  (current architecture, full file inventory, phases, new-repo decision, the
  denoise-shim interaction, doc corrections).
- Tagged every related note with [EC-FORK] + links: README (For Developers),
  LOTUS_BUGS (EC limitations), LOTUS_TODO (soundboard, denoise, soundboard
  cross-origin correction), LOTUS_FEATURES (call section).

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

LOTUS_TODO.md

@@ -264,6 +264,7 @@ Features:
 
 **What:** Grid of short audio clips playable into the call audio stream via Web Audio API (AudioBufferSourceNode → MediaStreamDestinationNode → mixed with mic). Built-in clips + user-uploadable custom clips (stored as mxc://). Accessible from call controls bar.  
 **[AUDIT REQUIRED]** Verify the Element Call integration exposes the mic MediaStream for mixing. This is the highest-risk part of this feature.  
+**🔱 [EC-FORK]** Owning the EC source (see [`HANDOFF_ELEMENT_CALL_FORK.md`](./HANDOFF_ELEMENT_CALL_FORK.md)) would unblock real audio-injection — a proper soundboard mixed into the call — which is impossible against the prebuilt bundle today.  
 **Complexity:** High.
 
 ---
@@ -281,6 +282,7 @@ Features:
 
 **What:** High-end background noise cancellation using a pre-trained ML model (RNNoise) running in the browser. Removes dogs, fans, and keyboard clicks from the mic stream.  
 **Shipped:** 3-tier setting (Off / Browser-native / ML) in Settings → General → Calls. ML tier injects a same-origin pre-init shim into the vendored Element Call `index.html` that monkeypatches `getUserMedia` and routes the captured mic through an RNNoise `AudioWorklet` before LiveKit publishes — no EC fork required. See LOTUS_FEATURES.md → "Noise Suppression (Advanced Multi-Tier)".  
+**🔱 [EC-FORK]** Once we own the EC source (see [`HANDOFF_ELEMENT_CALL_FORK.md`](./HANDOFF_ELEMENT_CALL_FORK.md)), denoise should become a first-class audio stage **inside** EC instead of an `index.html` getUserMedia monkeypatch — more robust, survives reconnects (fixes the A7 mic-after-reconnect bug), and removes the build-time injection hack.  
 **Key decision:** LiveKit's Krisp filter is LiveKit-Cloud-only (we self-host the SFU); EC's own RNNoise PR #3892 is unmerged. The shim is the same post-capture pipeline #3892 uses, executed from the realm we control, so it survives EC version bumps.  
 **AEC note (resolved-as-accepted):** WebAudio capture routing can weaken browser AEC — same tradeoff as EC's upstream feature; mitigated by keeping `echoCancellation`/`autoGainControl` on the raw capture and labeling the tier "beta".
 
@@ -531,6 +533,8 @@ Exhaustive, low-level implementation details for backlog items. Follow these pat
 - Pass the destination's `.stream` to the call bridge.
 
 > ⚠️ **[Gemini_Found — CORRECTED]** Gemini originally suggested using LiveKit's `LocalAudioTrack.replaceTrack()` to mix audio into the call stream. This is **not possible** from Lotus Chat's realm: Element Call runs in a **cross-origin iframe** controlled via `matrix-widget-api` (postMessage). LiveKit's JS SDK and its `LocalAudioTrack` live inside EC's sandboxed context — inaccessible from our code. This directly contradicts the confirmed constraint already listed in the Server Capabilities table: _"Cindy CANNOT inject audio into EC call stream — In-call soundboard must be redesigned as local-only."_ The soundboard must be a local-playback-only feature (output through the user's speakers, not mixed into the call audio stream).
+>
+> 🔱 **[EC-FORK — partial correction]** The "cross-origin" claim above is **outdated**: EC is now **same-origin** / self-hosted (`iframe.sandbox` has `allow-same-origin`; we read `contentDocument`). The _practical_ blocker still holds — LiveKit's `LocalAudioTrack` lives in EC's **module scope** (not on `window`), so it's unreachable from cinny even same-origin. **Owning the EC source** (see [`HANDOFF_ELEMENT_CALL_FORK.md`](./HANDOFF_ELEMENT_CALL_FORK.md)) is the path to a real call-audio-inject API, which would unblock a true in-call soundboard.
 
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