docs: add v0.3 architecture roadmap

[CoderDeltaLAN]

Summary

• Adds the v0.3 architecture and roadmap planning document.
• Defines the narrow v0.3 release train: output/exit-code contract, golden output foundation, doctor baseline, budget baseline, explain baseline, and release preparation.

Scope

• Documentation only.
• No code changes.
• No CI changes.
• No dependency changes.
• No release, tag, or PyPI changes.

Validation

• Local pre-stage checks passed.
• Staged review passed.
• Pre-push strong check passed.
• 118 unit tests passed locally with dev dependencies.
• Ruff passed.
• Text hygiene passed.
• Git whitespace checks passed.

Risk

• Low. Planning document only.
• Main risk is roadmap overreach; mitigated by explicit non-goals and no stable API or security claims.

Rollback

• Revert this documentation-only commit.