Bottom-panel Stage 2B-3, part 1 of 3: the compatibility-preserving v21
activation mechanism and the negotiated `panel_capable` flip.
2B-1 reserved the v21 wire and 2B-2 built the daemon projection behind
it, both dark, because the handshake is server-first: the daemon writes
`Hello` before the frontend has said anything, and a frontend rejects a
`protocol_version` outside its supported range *before* it can send
`AttachRequest`. Advertising 21 there is therefore an incompatible act on
its own, independent of whether one new message is ever exchanged.
So the advertised version does not move. `ADVERTISED_PROTOCOL_VERSION`
becomes a permanent compatibility BASELINE, and the session's real
version is settled one message later, by the frontend:
1. the daemon advertises the baseline (20, unchanged);
2. the frontend answers `requested_protocol_version(baseline)` — its
own `PROTOCOL_VERSION` when the baseline is the current one, and a
verbatim echo of anything older;
3. the daemon records `negotiated_session_version(offer)`.
A shipped v20 frontend echoes 20 and gets a v20 session, byte-for-byte
as before — the real-daemon acceptance that emulates its rejection point
still passes untouched. A current frontend offers up and gets v21. The
`Hello` encoding and value are unchanged, which is why the old frontend
never sees a version it must reject.
`peer_declared_panel_support` gains the arm 2B-2 deliberately left off:
a semantic session is panel-capable exactly when it negotiated
`PANEL_MIN_VERSION` or later. The gate is on placement, not only
transport, so a v6-v20 semantic session keeps the Stage 1 fallback.
The GPU client's `server_protocol_version` splits into
`session_protocol_version` (what the session speaks — every wire gate
keys on this) and `baseline_protocol_version` (what `Hello` advertised).
They now differ in the normal case, and that difference IS the
compatibility property, so both headless probe reports emit both keys and
the two ratchets that read them assert both directions: session 21 AND
baseline 20. Asserting only the session version would pass if the
baseline had been bumped too — the exact incompatible change this
mechanism avoids.
Also fixes a pre-existing `unused_mut` in a `crdt`-gated daemon test,
dark to the standard clippy gate because that gate runs without the
feature.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Lv428Fth9LRtffwJSsqH7T
Levi Neuwirth
d03845e826