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_FEATURES.md

@@ -322,6 +322,11 @@ Users can set a custom background color for `@mention` chips that highlight thei
 
 ## Voice / Video Call Improvements
 
+> 🔱 **[EC-FORK]** Element Call is embedded as a **pre-built npm bundle** today.
+> The plan to fork & self-build it from source for true ownership — and which of
+> the items below would move into our EC source — is in
+> [`HANDOFF_ELEMENT_CALL_FORK.md`](./HANDOFF_ELEMENT_CALL_FORK.md).
+
 ### Element Call Upgrade
 
 Upgraded embedded Element Call widget from **0.16.3** to **0.19.4**.