T11 - client automatic era detection

[kubinio123]

Target branch: 2026-07-28-protocol-support

Requires:

• T10 - client stateless lifecycle #248

A chimp client with no pinned protocol version works against 2026-07-28 servers and 2025-xx servers without configuration. Users who upgrade chimp and talk to a legacy server see no change.

Policy

• The client has a version policy, automatic detection is the default. A fixed version, as today, is still available and skips detection.
• With a fixed stateless version the client behaves as in T10. With a fixed legacy version it behaves as today.

Detection

• Over HTTP the first request is server/discover. A discover result, or a recognised 2026-07-28 error such as -32022, means the server is modern. Any other outcome, including a 400, 404 or 405 whose body is not a recognised modern error, means the server is legacy.
• Over stdio the first request is server/discover as well. A timeout also means legacy.
• The decision never depends on one specific error code.
• After a modern decision the client uses the stateless wire and negotiates the version on -32022 as in T10. After a legacy decision the client sends initialize with the highest legacy version chimp supports.
• The decision is made once per client and reused for its lifetime.

Errors

• A modern server that shares no version with chimp fails the client with an error that names both version lists.
• A legacy server that rejects initialize fails the client as it does today.

Coverage

• The rules above hold for the sync and streaming clients, over HTTP and stdio, on ZIO, Ox and Pekko.
• The existing integration tests against the legacy everything server pass with the default policy and no other change.
• Unit tests cover every row of the spec compatibility matrix that involves a chimp client: modern, legacy and dual-era client against modern and legacy servers.

Spec:

• Versioning - Backward Compatibility
• Versioning - Compatibility Matrix
• stdio - Backward Compatibility
• Streamable HTTP - Backward Compatibility

Conformance scenarios that must pass after this task: none new.

Docs: yes.

• docs/client/protocol.md: the detection rules, what the client sends first, and how to pin a version.