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, 7 insertions, 2 deletions

diff --git a/packages/meshbay-node/tests/test_task_lifetime.py b/packages/meshbay-node/tests/test_task_lifetime.py
index 3a5b8a5..9dffedb 100644
--- a/packages/meshbay-node/tests/test_task_lifetime.py
+++ b/packages/meshbay-node/tests/test_task_lifetime.py
@@ -245,8 +245,13 @@ def test_chunks_wait_for_room_on_the_channel(session):
     work through the rest — which is what "stuck at 1 MB" looks like, one chunk
     being exactly one megabyte.
     """
-    fn = session[session.index("async def _do_file_request"):]
-    fn = fn[:fn.index("\n    def _do_stream_segment")]
+    start = session.index("async def _do_file_request")
+    # Up to whatever the next member is. This used to end at
+    # "\n    def _do_stream_segment" — a neighbour removed in MNP 2.0 — and an
+    # `index()` on a name that no longer exists fails the test for a reason
+    # that has nothing to do with what it is about.
+    nxt = re.search(r"\n    (?:@|(?:async )?def )", session[start:])
+    fn = session[start:start + nxt.start()] if nxt else session[start:]
     assert "DOWNLOAD_BUFFER_HIGH" in fn, "the send buffer has to be watched"
     assert "await asyncio.sleep" in fn, "waiting for room is the point"
     assert 'readyState != "open"' in fn, (