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, 38 insertions, 10 deletions

diff --git a/packages/meshbay-node/tests/test_security_regressions.py b/packages/meshbay-node/tests/test_security_regressions.py
index 1a318f7..fa821d9 100644
--- a/packages/meshbay-node/tests/test_security_regressions.py
+++ b/packages/meshbay-node/tests/test_security_regressions.py
@@ -743,20 +743,48 @@ def test_pre_handshake_message_budget_is_small():
         list(buf.messages())
 
 
-def test_stream_segment_is_not_synchronous():
+def test_no_transport_ships_media_outside_the_aead():
     """
-    H6: _do_stream_segment ran subprocess.run(timeout=30) inside the event loop,
-    stalling every peer on the node for up to thirty seconds per request.
+    `stream_seg` served an MPEG-TS segment as base64 with no encryption at all
+    — the one content-plane message that never went through a GEK-derived key,
+    on both transports, answering any authenticated member. Its browser caller
+    was defined and never invoked. Removed in MNP 2.0 rather than repaired:
+    `stream_data` already does the job under `chunk_ciphertext`.
 
-    Asserts the property (the worker is a coroutine, ffmpeg is spawned through
-    asyncio) rather than grepping for "subprocess.run" — which also matches the
-    comment that documents the old behaviour.
+    Asserted as the property, not as "the function is gone": what matters is
+    that no transport has a field carrying media bytes past the AEAD. The old
+    H6 test lived here — it pinned `_do_stream_segment_async` to a coroutine so
+    ffmpeg could not block the event loop — and the handler outliving that
+    concern is exactly what this replaces.
     """
-    import ast
-    import inspect
-    from meshbay_node.transport.webrtc_server import WebRTCPeerSession
+    import re
+
+    from meshbay_common.protocol import MNP
+
+    assert not hasattr(MNP, "STREAM_SEGMENT"), (
+        "the constant outliving the handlers is how a deleted endpoint keeps "
+        "looking like part of the wire contract")
+
+    root = Path(__file__).parent.parent / "src" / "meshbay_node" / "transport"
+    for name in ("webrtc_server.py", "quic_server.py", "quic_client.py"):
+        source = (root / name).read_text(encoding="utf-8")
+        # Word boundaries: `_stream_segments` and `STREAM_SEGMENT_SIZE` belong
+        # to the live `stream_data` path, which is encrypted and stays.
+        assert not re.search(r"\bstream_seg\b", source), (
+            f"{name} still speaks stream_seg")
+        assert not re.search(r"\bSTREAM_SEGMENT\b", source), (
+            f"{name} still names the removed type")
+        assert "data_b64" not in source, (
+            f"{name} carries a base64 media field — media leaves this node "
+            "encrypted or not at all")
 
-    assert inspect.iscoroutinefunction(WebRTCPeerSession._do_stream_segment_async)
+
+def test_ffmpeg_never_blocks_the_event_loop():
+    """
+    H6, the half that survives `stream_seg`: the live streaming path still
+    spawns ffmpeg, and a synchronous spawn stalls every peer on the node.
+    """
+    import ast
 
     source = (Path(__file__).parent.parent / "src" / "meshbay_node"
               / "transport" / "webrtc_server.py").read_text(encoding="utf-8")