[codex] Add Flavor B concept index design

[rdhyee]

Summary

• add a read-only investigation/design note for iSamples Explorer issue #256
• document that the four external Getty/UBERON acceptance URIs do not currently resolve to IdentifiedConcept nodes in the queried PQG files
• propose a separate sample_concepts sidecar parquet for Flavor B described-by URI search

Validation

• inspected remote/current wide PQG schema and local cached wide/narrow PQG files with DuckDB
• confirmed direct MaterialSampleRecord -> IdentifiedConcept paths for keywords and category relations
• verified the staged diff is docs-only

No frontend or deployment changes were made.