Start MPEG-TS subscribers on safe stream boundaries

[jake1164]

Child of #62.

Goal: avoid handing Jellyfin/FFmpeg stream data that begins mid-packet or mid-H.264 decoder context.

Context:

• FFmpeg logs from 2026-04-24 showed Invalid data found when processing input, then non-existing PPS 0 referenced, decode_slice_header error, and Non-monotonic DTS warnings.
• Current shared stream attach path starts viewers at live edge in src/M3Undle.Web/Streaming/Subscribers/SubscriberConnection.cs.
• Upstream reads are chunked in src/M3Undle.Web/Streaming/Sessions/ChannelStreamSession.cs; default chunk size is not MPEG-TS packet aligned.

High-level ask:

• Preserve 188-byte MPEG-TS packet alignment.
• Attach new subscribers from a sync-safe point.
• Prefer PAT/PMT and IDR/SPS/PPS-aware startup where practical.