[~] #820 fix active_connection_id_limit: only exclude self-generated CID per RFC 9000 §18.2

[dreamwind1985]

Summary

Per RFC 9000 §18.2, active_connection_id_limit is the maximum number of connection IDs from the peer that an endpoint is willing to store, and it includes the connection ID received during the handshake.

The previous fix (#777, commits 26f2c50 and 1db5fd5) incorrectly excluded all handshake CIDs from the limit, citing a non-existent RFC §5.1.1 quote. This caused xquic to issue too many NEW_CONNECTION_ID frames, exceeding the peer's actual limit and triggering CONNECTION_ID_LIMIT_ERROR in nginx, ngtcp2, quiche, etc.

Root Cause

xquic's dcid_set contains two handshake CIDs:

1. The client's self-generated initial DCID — NOT from the peer, should NOT count toward the limit
2. The server's SCID from the Initial response — IS from the peer, should count toward the limit

The previous fix excluded both, but only #1 should be excluded.

Fix

• Redefine original_cid_cnt to track only self-generated CIDs (not from the peer), not all handshake CIDs
• Remove the mark_original call for the server's SCID (pkt->pkt_scid) in xqc_conn.c, since it IS from the peer and counts toward the limit
• Update countable_cnt comments to reference RFC 9000 §18.2 correctly

Verification

After fix, for active_connection_id_limit=8 (nginx default):

• countable = (unused + used) - original_cid_cnt = (7 + 2) - 1 = 8 == limit
• nclient_ids = 1(handshake) + 7(NCID) = 8 <= 8 ✓

Interop test results (xquic vs nginx): 14/16 pass (2 unrelated failures, 2 unsupported)

Closes #820