Decide external-model degrade strategy for constrained decoding (#10)

[vbergeron]

Adds docs/adr/0001-external-model-decoding-strategy.md, the written decision
issue #10 asked for.

Per call site:

• Gate.label/1 (closed 3-word vocabulary) -> Option B: provider structured
  output / tool-use enum. A real guarantee, not best-effort, so no new
  degraded tier is needed there; falls back to classify_lexically/1 exactly
  as the local path does today.
• Turn.extract/4 and Turn.query/4 (open-ended Prolog) -> Option A: strong
  system prompt + bounded validate-and-retry against the existing Clause
  parser (split/1, rejection/1). No provider primitive can constrain
  arbitrary Prolog, so this is the new "best-effort external,
  retry-validated" tier the issue anticipated.

Also decides where the retry loop lives (Turn/Gate, behind a new
Model.constrained?/0 capability query — not inside Model or a backend
module, keeping the #9 seam free of Prolog-specific logic), how the
weakened guarantee is surfaced (a syntax_guaranteed handshake field and a
new prolog_retry_exhausted error code, both additive per PROTOCOL.md's own
versioning rule), and what happens to the unqualified GBNF-guarantee claims
in README.md and Manifold.Clause's moduledoc.

No code changes: this is the design decision itself. Implementation is
follow-up work, as #10 specifies. README gets a one-paragraph pointer to the
ADR under "Status / next steps" so it's discoverable.

Co-Authored-By: Claude Sonnet 5 noreply@anthropic.com
Claude-Session: https://claude.ai/code/session_01J2hHjE9kH28XhE5Ksj7YES