Align docs and behavior: address purpose, empty results, validation

[frankfarach]

Summary

• Normalize address_purpose inputs to API uppercase values and add coverage.
• Return a typed npi_results object for zero‑record searches.
• Vectorize npi_is_valid() for multiple NPIs.
• Clarify taxonomy fallback behavior in npi_summarize docs and README.
• Add Author/Maintainer fields for R CMD check.
• Update NEWS.md and refresh roxygen output.

Changes

• npi_search() accepts case‑insensitive address_purpose and normalizes to LOCATION/MAILING/PRIMARY/
  SECONDARY.
• Zero‑record searches return a structured npi_results.
• npi_is_valid() returns logical vectors for vector inputs.
• npi_summarize documentation notes fallback to first taxonomy if none marked primary.
• NEWS.md updated; roxygen regenerated (RoxygenNote updated).

Testing

• R CMD build .
• R CMD check npi_0.2.0.9000.tar.gz

Notes

• No behavior changes beyond the described alignments; existing pipelines should remain compatible.