docs: clarify repository scope boundary

[stacknil]

Summary

• add a root README Scope Boundary section after the supporting spatiotemporal diagnostics section
• distinguish precipitation-anomaly-diagnostics as the compact reviewer-facing mini-lab
• distinguish precipitation-anomaly-diagnostics-lab as the extended lab variant with configurable diagnostics utilities
• repeat in both precipitation README first screens that these are supporting scientific-data mini-labs, not part of the sbom-diff-and-risk release surface or a separate meteorology portfolio

Validation

• git diff --check
• README privacy/sensitive-pattern scan: no known identifiers, education/course terms, local paths, or secret-like strings

Notes

Documentation-only clarification to reduce maintenance ambiguity and keep the repository narrative centered on deterministic review infrastructure.