Config reference

Key	Env	Default	Meaning
`nexus.us_tax_data.location`	`NEXUS_US_DATASET_LOCATION`	the public mirror URL	http(s) base URL or local directory holding `by-section/nexus.json`
`nexus.us_tax_data.ttl`	`NEXUS_US_DATASET_TTL`	`86400`	seconds a fetched section is cached
`nexus.approaching_ratio`	`NEXUS_APPROACHING_RATIO`	`0.8`	fraction of a threshold that flags a state "approaching"

Point location at a pinned dataset tag or a committed local copy for an offline/deterministic build; an unreachable location denies (no threshold), never guesses.