Roadmap: 1.0 (Foundation Models 26) and 2.0 (Foundation Models 27)

Now that @pcuenca and I are back on this, here's the plan:

**1.0** — a stable release against the current Foundation Models API in iOS/macOS 26. Before tagging it, I want to tidy up the places where AnyLanguageModel grew beyond what Apple's framework offers, mostly to compensate for early limitations:

- Remove the APIs that are already marked deprecated (Llama passthrough properties, the legacy Gemini `Thinking` / `ServerTool` types).
- Fix `respond(to:schema:)`, which currently ignores its `schema` argument.
- Make `Instructions` `Sendable`, and close a few other small parity gaps.
- Clearly mark our extensions (tool execution delegate, image inputs, `[custom:]` options) as extensions.

**2.0** — track the Foundation Models changes in iOS/macOS 27. I expect source-breaking changes around session construction, image inputs, and generation options, so that'll be a major version. Availability-gated additions (like #206) can land before then.

If there's something you need in 1.0, please say so here.

Start by reviewing the roadmap's listed Foundation Models API parity gaps and the current 1.0 and 2.0 compatibility plans. No files, tests, or entry points are named in the issue; done requires an agreed scope for the 1.0 cleanup and the iOS/macOS 27 breaking changes.

Written by the indexing model from the issue text.