recordings: capture session:transport-gap so a replay shows the gap row (#1381 follow-up)

[Juliusolsson05]

Follow-up of #1381 / PR #1442.

Session recordings (SessionRecorderManager's channel list, recordedSession, reconstructSlices) do not capture session:transport-gap, nor the transportGaps field of the initial history chunk. A recording of a session that lost a proxy span therefore replays without the durable "not captured" row the user saw.

Fix shape:

• add the channel to the recorded set;
• fold it into the reconstructed runtime's transportGaps;
• pin it with a replay test.

🤖 Generated with Claude Code

[Juliusolsson05]

Blocked (W2): session:transport-gap and the chunk's transportGaps exist only on W3's open #1442, not on origin/main 6dd23a49. Suggest W3 takes it after #1442 merges (its follow-up, same types).