feat(rich-content): deliver R0-R4 rich content foundation

[knqiufan]

Summary

• Deliver R0-R4: versioned content blocks, artifact storage, local previews, charts, and local GeoJSON maps.
• Keep rich-content writes and rendering behind default-off feature flags.
• Add implementation plans, phase records, and R0-R4 handoff evidence.

Validation

• Remote CI run #31288456569 completed successfully (8/8 checks).
• Phase-specific CI evidence is documented in docs/planning/rich-content-delivery/09-r0-r4-handoff.md.