feat(mnp)!: seal the upload under the group key

Downloads have been encrypted under a GEK-derived key since the beginning:
`file_chunk` and `stream_data` both go through `chunk_ciphertext`. Uploads
never were. `file_upload` carried the filename and the raw bytes in plain
msgpack, and `file_upload_ack` carried the name the node stored them under —
so the same file was ciphertext leaving a node and plaintext arriving at one.
There was no threat model behind that asymmetry.

Both halves now travel sealed under a third groupbox purpose,
HKDF(GEK, info="meshbay:upload:v1"). The filename, the destination folder and
the bytes are all inside the seal; only `upload_id` and `chunk_index` stay in
clear, because the node routes and orders on them before it can decrypt. This
direction seals *towards* the node — it holds the GEK for its own group — and
it opens the payload before it picks a destination or touches the disk.

What that forced, and why none of it is optional:

  - `filename` was the correlation key on both sides. It cannot be: matching an
    ack to its request by name would hand back exactly what the seal hides.
    `upload_id` replaces it — client-drawn, opaque to the node, unique within a
    connection, never an authorization input. The property it guarded (one
    refusal fails one upload, not every upload in flight) is unchanged.
  - Refusals can no longer quote what they refused. `No directory named 'X'`
    becomes `No such directory in this group` plus the `code` that was already
    there; the client knows what it sent.
  - No plaintext fallback. A path that still accepts plaintext is not a sealed
    path, so an unsealed `file_upload` is refused with `upload_not_sealed`.

Hardened while here, because what comes out of a seal is authenticated but not
validated — a member can seal anything: `filename` and `data` have their types
checked before any upload state is created, and `chunk_index`/`total_chunks`,
which are outside the seal by necessity, can no longer raise where a refusal
was meant.

Tests. `test_upload_sealed.py` pins the node half: nothing identifying on the
wire, tamper/wrong-key/wrong-group all refused with nothing written, and
multi-chunk reassembly unchanged. `test_upload_seal_client.py` drives the
shipped `uploadFile` over the shipped `crypto.js` under node and feeds its real
frames to the real `_do_file_upload` — the file lands intact, and the ack the
node actually produced comes back with the name it chose for a collision, which
is the half a source-reading test cannot see. Both upload purposes join the
JS/Python groupbox parity vectors.

BREAKING CHANGE: MNP 2.0. `file_upload`/`file_upload_ack` change shape on the
wire every deployed client speaks, which is MAJOR by the same rule 1.0 was —
but the break is confined to uploads. `MNP_MIN_SUPPORTED` stays at "1.0", so a
1.x peer still connects, browses, downloads, streams and chats; only its
uploads are refused, with a message saying which side is old. The client checks
the node's version before sending a chunk, so neither side meets this as a
timeout. This is the version negotiation shipped in 1.0 earning its keep: 1.0
cost a flag day, 2.0 costs a refusal code.

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

0 files changed, 0 insertions, 0 deletions