feat: add public source immutable landing slice

[zhouning]

Summary

• stage verified public/open bytes into a content-addressed immutable local Landing
• atomically register Resource, ResourceVersion, and input Artifact through the existing platform gateway
• expose the same-tenant authenticated landing endpoint and add CI/CD coverage
• record ADR-081, Natural Earth smoke evidence, and roadmap/system-of-record updates

Verification

• uv run ruff check on changed Python files
• uv run pytest -q data_agent/test_public_source_landing.py data_agent/test_public_source_landing_postgres.py: 6 passed, 1 skipped locally
• PostgreSQL integration: 1 passed in prior Docker verification
• required platform suite: 735 passed in prior verification
• Natural Earth public-domain ZIP stage/replay/verify smoke passed
• git diff --check and evidence JSON validation passed

Boundary

• production_ready remains false
• protected Chongqing admission remains blocked and unchanged