docs: choose the Python and Rust API boundary

[jt55401]

Summary

• adds a decision document with four scope options for the Python and Rust bindings
• records the current capability matrix across all five languages
• separates payload construction, verification, resolution, and private-key signing

Recommendation

Choose option 1 for v0.3.0: retain the deterministic canonicalizer boundary and document it prominently. Revisit payload construction when a Python or Rust publisher has a concrete owner and use case.

Review requested

Comment with Decision: option 1, option 2, option 3, or option 4, plus any capability that must block v0.3.0.

Validation

• make check
• git diff --check

AI assistance: Codex drafted the document and checked its claims against the current binding APIs, manifests, and shared conformance suite.