feat(transport): add EmbeddedTransportBackend for no-iframe embedding

[mishraditi]

Description:
Adds EmbeddedTransportBackend - a new ITransportBackend that communicates through direct function calls instead of postMessage across an iframe.

It sits here alongside PostMessageTransportBackend since this is where the transport layer and ITransportBackend interface already live.

Changes:

• EmbeddedTransportBackend.ts - new backend, use static createPair() to get a linked pair of instances.
• EmbeddedTransportBackend.spec.ts - 12 unit tests covering pairing, bidirectional messaging,
  dispose, and Transport integration.
• transport/index.ts - exports EmbeddedTransportBackend alongside the existing backends.

This is part of the "External API without iframe" project. Follow-up PRs in jitsi-meet will use this to render Jitsi Meet directly into a host page div without an iframe.

[mishraditi]

@saghul @hristoterezov,
Closing this, after the discussion on jitsi/jitsi-meet#17477 the embedded API doesn't use a transport anymore. Both sides run in the same page and call each other directly, so this loopback backend isn't needed.