[MatrixRTC] Multi SFU support + m.rtc.member event type support

[toger5]

What changed

• New event type:
  • Add m.rtc.member event type.
  • Add tests covering m.rtc.member.
• CallMembership:
  • CallMembership can now be constructed from m.rtc.member and exposes the same API shape as m.call.member to the RTC session.
  • Move “oldest member” selection logic into CallMembership (was previously in the membership manager). This removes unnecessary coupling: the RTCSession passes the context required by CallMembership directly.
• Membership manager (RTC session):
  • Add useRtcMemberFormat to control sending m.rtc.member instead of m.call.member.
    • Note: this may be merged with useStickyEvent in a follow-up when we consolidate the rollout.
• Naming cleanups:
  • Rename “focus” to “transport”.
  • Rename “session” to “slot”.
• Active (in use) transport resolution:
  • Remove getFocusInUse.
  • Rename MatrixRTCSession.getActiveFocus → MatrixRTCSession.resolveActiveFocus.
  • Use the new CallMembership API to compute the active transport (BREAKING).
• Joining a session:
  • Change the second argument from activeFocus: Focus to multi_sfu_focus: Transport.
  • Passing a defined multi_sfu_focus implies multi-SFU mode; undefined falls back to the oldest-membership selection.

Breaking changes

• MatrixRTCSession:
  • Remove getFocusInUse().
  • Rename getActiveFocus() to resolveActiveFocus().
• Join API:
  • Old: join(sessionId, activeFocus: Focus)
  • New: join(sessionId, multi_sfu_focus?: Transport)
    • Defined multi_sfu_focus → use multi-SFU with the provided transport.
    • undefined → legacy behavior using the oldest membership.

Todos for consumers updating the PR (currently only known: Element Call):

• Replace calls to getActiveFocus() with resolveActiveFocus().
• Remove any usage of getFocusInUse().
• Update callers of join(...) to pass multi_sfu_focus only when opting into multi-SFU.

Backward compatibility and rollout

• Reading: CallMembership can handle both m.call.member and m.rtc.member, so existing rooms continue to work.
• Emitting: gated by useRtcMemberFormat:
  • false (default): continue to emit m.call.member.
  • true: emit m.rtc.member.
• Multi-SFU usage is opt-in via the new join(sessionId, multi_sfu_focus?: Transport) signature.

Checklist

• Tests written for new code (and old code if feasible).
• New or updated public/exported symbols have accurate TSDoc documentation.
• Linter and other CI checks pass.
• Sign-off given on the changes (see CONTRIBUTING.md).

[robintown]

I think this looks like a good first shippable iteration!

Both Timo and I have given our takes on all outstanding comments and I agree that they've been properly addressed.