refactor(mnp)!: remove stream_seg, the last unencrypted content message

`stream_seg` answered with an MPEG-TS segment as base64 with no encryption at
all — the one message on the content plane that never went through a
GEK-derived key. Live on both transports, answering any authenticated member.

It predates `stream_data`, which does the same job properly (`chunk_ciphertext`,
keyed per segment, AES-256-GCM) and has since Phase 12. Its only browser caller,
`fetchStreamSegment`, was defined and never once invoked — a plaintext media
endpoint with no client. Removed rather than repaired.

Gone with it: `_extract_segment` and the ffmpeg semaphore in quic_server, the
`fetch_stream_segment` QUIC client method, and `_b64decode` in transport.js,
which had no other caller.

The H6 regression test lived on this handler — it pinned `_do_stream_segment_async`
to a coroutine so `subprocess.run` could not stall the event loop for thirty
seconds per request. It is replaced by the property that outlives the handler:
no transport carries media outside an AEAD, asserted on `stream_seg` and
`data_b64` across all three transport modules. The half of H6 that survives —
the live streaming path still spawns ffmpeg — keeps its own test.

BREAKING CHANGE: `stream_seg` is no longer answered on either transport. No
shipping client sends it. Recorded as part of MNP 2.0, whose other half — the
sealed upload — carries the version bump.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01AsoWC3GmhNdwVFomW3QjH3

1 files changed, 0 insertions, 30 deletions

diff --git a/packages/meshbay-node/tests/test_webrtc_transport.py b/packages/meshbay-node/tests/test_webrtc_transport.py
index dc74752..88b6f63 100644
--- a/packages/meshbay-node/tests/test_webrtc_transport.py
+++ b/packages/meshbay-node/tests/test_webrtc_transport.py
@@ -743,36 +743,6 @@ async def test_webrtc_peer_cleanup_on_close(sk_node, sk_hub, gek, shared_dir):
 
 
 @pytest.mark.asyncio
-async def test_webrtc_stream_segment_missing_file(sk_node, sk_hub, gek, shared_dir):
-    """WebRTC DataChannel: stream_segment for non-existent file returns error."""
-    hub_pk_pem = _hub_pk_pem(sk_hub)
-    indexer = DirectoryIndexer(roots=one_root(shared_dir), group_id="g", sk_node=sk_node, gek=gek)
-    await indexer.initial_scan()
-
-    transport = WebRTCTransport(
-        sk_node=sk_node, hub_pk_pem=hub_pk_pem, gek=gek,
-        roots=one_root(shared_dir), index=indexer.index,
-        stun_servers=[],
-    )
-
-    browser_pc, channel, received = await _setup_peer(
-        transport, sk_hub, gek, "peer-stream")
-
-    channel.send(_pack({
-        "type": MNP.STREAM_SEGMENT, "v": MNP_VERSION,
-        "file_id": "nonexistent-file-id",
-        "segment_index": 0, "segment_duration": 4,
-    }))
-
-    msg = await asyncio.wait_for(received.get(), timeout=5.0)
-    assert msg["type"] == "error"
-    assert "not found" in msg["detail"].lower()
-
-    await browser_pc.close()
-    await transport.close_all()
-
-
-@pytest.mark.asyncio
 async def test_webrtc_wrong_gek_proof_rejected(sk_node, sk_hub, gek, shared_dir):
     """WebRTC DataChannel: wrong GEK proof is rejected — hub admin can't fake membership."""
     hub_pk_pem = _hub_pk_pem(sk_hub)