docs: refine public positioning for precipitation diagnostics

[stacknil]

Summary

• Rename the root README project entry so sbom-diff-and-risk remains the flagship tool and precipitation diagnostics is framed as a supporting spatiotemporal diagnostics project.
• Rework the precipitation diagnostics README opening around reproducible workflow capability instead of first-screen domain specificity.
• Add PUBLICATION_BOUNDARIES.md and a reviewer path for data, publication, methodology, inference, report, and synthetic-test review.
• Align package metadata with the supporting spatiotemporal diagnostics positioning.

Validation

• python -m pytest in projects/precipitation-anomaly-diagnostics (6 passed, local Python 3.14)
• python -m compileall src scripts
• git diff --cached --check
• High-risk privacy scan for real institution/path/secret-like patterns; no matches

Notes

• This is a documentation and metadata positioning PR only.
• It does not modify tools/sbom-diff-and-risk or its release surface.