refactor: retire legacy v0/v1/v2 media tiers and the base64 JPEG fallback

Pre-alpha with no deployed peers, so the compatibility surface collapses
to the two paths the product actually uses: upstream MoQ (preferred) and
grouped-v3 (sole fallback tier).

Host: MEDIA_ALPN_V1/MEDIA_ALPN_V2 are no longer registered; serve_media
and serve_media_v2 (all of media_v2.rs) are deleted, with
media_frame_for_encoded moved next to its only consumer in moq.rs.

Client: the hardcoded use_v1 flag and the unreachable v0 leg (no hello,
single start byte) are gone, along with the v2/v1 fallback ladder, the
v1 object-reorder machinery, and the LegacyV0/ReliableStreamV1/
IndependentObjectsV2 transport variants. The negotiated control stream
is now threaded as a plain (non-Option) value.

Base64 fallback: per AGENTS.md 'Never reintroduce base64 media events;
use binary Tauri channels', the openh264-to-JPEG-to-base64 app.emit
path is deleted end to end - Rust decoder branch, FramePayload event,
--dev-force-jpeg flag, openh264/image/base64 dependencies, the portal
JPEG presenter, and the 'frame' event listener. Connecting now fails
fast with a clear error when the webview lacks WebCodecs H.264 support.

Probe: v1/v2 tiers, their CLI flags, and the duplicated v1 reorder
state machine are removed (-520 lines).

Protocol: MEDIA_ALPN_V1/V2, LEGACY_FRAME_ALPN_V0/LEGACY_INPUT_ALPN_V0,
and the v1/v2 stream IO (read/write_media_frame, read_media_object) are
deleted; MediaFrame stays as the MoQ object payload format.

The portal transport allowlist now mirrors the two remaining transports.

commit

5f090adffd773ab596726969758c545128078cf4

parent

d90cfb4b35cffc432ad92832997bf5c1b2c580d6

author

Dami <dami.etoile@gmail.com>