Merge origin/main into the chat encryption work

Both sides landed a breaking MNP change and both called it 2.0, which is right:
the sealed upload, the removal of `stream_seg` and mandatory chat encryption
share one flag day. They are recorded as one version in `__init__.py` rather
than as a race between two.

The resolutions that were decisions rather than mechanics:

* **`MNP_MIN_SUPPORTED` moves to "2.0".** The sealed upload alone was a
  *confined* break — a 1.x peer could still connect, browse, download, stream
  and chat, with only its uploads refused by `upload_not_sealed` — so the floor
  deliberately stayed at "1.0". Mandatory chat encryption ends that
  confinement: a 1.x peer can neither produce a sealed chat message nor read
  one, so it would connect, look fine, and be unable to say anything. Refusing
  it at the handshake is the honest form. The per-message `upload_not_sealed`
  path is untouched and still right if the floor is ever lowered.
* **`sendChat` throws on an `error` reply**, from origin, applied to the sealed
  send. It matters more after this change, not less: the node now refuses a
  stale epoch, a malformed envelope and a device claim that is not the
  connection's own, so there are three new ways for a message to be rejected
  and none of them may look like a message that was sent.
* **`req_id` supersedes the per-type routing** this branch added for
  `chat_keys_resp` and `device_hello_ack`. Both blocks are kept beside the
  existing `chat_hist_resp` one, for the same stated reason — a node too old to
  stamp — and their comments no longer claim to be the mechanism that closes
  the class. `req_id` is.
* **`chat_send_probe.py` is rebuilt on origin's structure**, not beside it: two
  scenarios, a stub that stamps `req_id`, `music_meta_req` as the older pending
  request. The encrypted path is layered on — a real Ed25519 device key
  generated in the page, and a `chat_keys_resp` sealed by the shipped Python,
  because a payload the page built itself would prove only that the page agrees
  with the page.
* **`test_reply_correlation.py` now sends a sealed message.** Its subject is
  which of the two messages leaving that handler carries the id; plaintext chat
  was only the fixture, and the node refuses one now.
* `groupbox` keeps both new purposes (`upload`, `chat_keys`); `protocol.py`
  keeps origin's removal of `STREAM_SEGMENT` and this branch's correction of
  the "Double Ratchet message" comment on `CHAT_MESSAGE`, which was wrong when
  it was written and is wrong differently now.

Full suite on the merged tree: 1993 passed, 11 failed — the same 11 that fail
on a pristine checkout (2 Windows service tests, 1 apps-enabled policy, 7
transcode tests that pass in isolation, and the WebRTC invite test that hangs
on its own).

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TZZxYjz8YeWRz13xDi8LJr

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