transport.Config used to carry a flat union of video+vp8+sei tuning fields
that every transport ignored except its own. Replace with an opaque
transport.Options marker interface and per-transport Options structs
(videochannel.Options, vp8channel.Options, seichannel.Options). Datachannel
keeps an unset Options.
link.Config gains TransportOptions and drops the 16 transport-specific
fields. server.Config and client.Config follow suit. session.Config is
left untouched in this commit — buildTransportOptions packs its existing
flat fields into the typed Options bundle before calling server/client
(session.Config is rebuilt in a later commit when YAML config moves to
typed sections).
Tests that synthesized link/server/client/transport configs are updated
to pass typed Options bundles. The shared e2eTransportOptions helper
replaces three copies of the flat field bundle in e2e/tunnel_test.go.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
