chore(canopy-notices): PDF/A conformance for archival-grade notice PDFs

For long-term archival (Pub 1075 records retention / state records schedules), generated notice PDFs should conform to PDF/A. The underlying typst-pdf crate doesn't currently support PDF/A output — conditional on upstream support landing. Monitor typst-pdf releases for the feature.

Extracted from typst-document-generation.adoc § Potential Improvements.