[codex] add figma metadata shard capture

[bigsmartben]

Summary

Adds a bounded Figma metadata shard capture utility and wires it into the intake visual-design workflow. The change also tightens IA-matrix preview readiness, blocks helper artifacts from being used as source-of-truth refs, and keeps docs, schemas, validators, templates, manifest defaults, and tests aligned.

Impact

• Figma get_metadata responses can be staged from local shard files into canonical metadata shards, an index, and node inventory without sending large provider payloads through agent context.
• Preview readiness now requires fused page/component IA matrix structure and interaction refs.
• Visual requirements/spec validators reject evidence packets, preview HTML, screenshots, and visual diffs in source/evidence ref fields.
• Release metadata is aligned for 0.1.5: manifest version, README release URL, and changelog entry.

Validation

• C:\Users\24598\.cache\codex-runtimes\codex-primary-runtime\dependencies\python\python.exe -m pytest -q -> 79 passed
• git diff --check -> passed (only Windows LF/CRLF warnings)

Release Notes

Prepared as 0.1.5 with changelog date 2026-07-03.