fix: use default PBKDF2 KDF instead of custom Rust HKDF for E2EE

The custom HKDF=1 path in the Rust FFI fork (EC-compat-changes)
produces different derived keys than the JS SDK's libwebrtc C++.
Switching to PBKDF2=0 lets libwebrtc's built-in C++ FrameCryptor
handle key derivation identically to how the JS SDK does it.

Also aligned ratchet_window_size=0 and key_ring_size=16 to match
Element Call JS SDK defaults (were 10 and 256).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

voice.py

@@ -441,20 +441,21 @@ async def _confluence_recent_pages(limit: int = 5) -> list[dict]:
 
 
 def _build_e2ee_options() -> rtc.E2EEOptions:
-    """Build E2EE options — Rust FFI applies HKDF internally (KDF_HKDF=1).
+    """Build E2EE options matching Element Call / LiveKit JS SDK defaults.
 
-    Pass raw base keys from Matrix key exchange events directly to set_key().
-    The Rust FFI derives the AES frame key via HKDF(base_key, ratchetSalt, ...) internally.
-    Element Call uses: ratchetWindowSize=10, keyringSize=256, ratchetSalt="LKFrameEncryptionKey"
-    NOTE: proto value 0 = PBKDF2 (NOT raw) — must use KDF_HKDF=1.
+    Use PBKDF2=0 (default, no custom Rust KDF) so libwebrtc's C++ FrameCryptor
+    handles key derivation the same way as the JS SDK. The custom HKDF=1 path
+    in the Rust FFI fork uses different output sizes, causing DEC_FAILED.
+
+    Element Call uses: ratchetWindowSize=0, keyringSize=16, ratchetSalt="LKFrameEncryptionKey"
     """
     key_opts = rtc.KeyProviderOptions(
         shared_key=b"",  # empty = per-participant mode
-        ratchet_window_size=10,
+        ratchet_window_size=0,
         ratchet_salt=b"LKFrameEncryptionKey",
         failure_tolerance=10,
-        key_ring_size=256,
-        key_derivation_function=KDF_HKDF,  # Rust FFI applies HKDF; we pass raw base keys
+        key_ring_size=16,
+        key_derivation_function=0,  # PBKDF2=0 = use libwebrtc default (no custom Rust KDF)
     )
     return rtc.E2EEOptions(
         encryption_type=rtc.EncryptionType.GCM,