feat: redesign Decision API

[rlippmann]

What changed

• Replace mapping-based Decision results with immutable typed variants.
• Add machine-readable semantic failure classifications, failed directives, and advisory canonical repairs.
• Strengthen conformance fixtures and public API contract coverage.
• Update the REPL JSON contract to version 2 with structured Decision fields.
• Cover all user-facing REPL paths, including human semantic errors and JSON no-directive results.

Why

• Let hosts consume semantic outcomes without parsing human-readable messages.
• Preserve explicit repair submission through the canonical engine boundary.
• Keep Python and future TypeScript implementations aligned on observable behavior.
• Ensure the machine-readable CLI exposes the complete Decision contract.

Closes #199

Checklist

• pre-commit run --all-files
• pytest
• REPL contract coverage gate: 100%