Rethink blocks_species: carry barrier ingredients, classify access late + per-context; add evidence-based dam-override #201

Motivation

blocks_species text[] (computed once in lnk_barriers_unify) bakes a binary per-species blocks/doesn't onto every barrier at unify time. Two problems surfaced in #196 / #200 (see RUNBOOK.md §5):

1. It conflates two orthogonal axes bcfp keeps separate: natural access (per-species, gradient-typed, observation/habitat-overridden) vs. anthropogenic descriptor (dam/pscis/modelled, passability-typed). Dams get baked into access wrongly.
2. It's computed before the override is known, so the "fish above ⟹ passable" evidence (observations/habitat) can't subtract from it.

bcfp has no binary blocks predicate — it builds per-species access sets from ingredients (gradient class, falls, subsurface) + evidence overrides, and tracks anthropogenic barriers separately. "Nothing is black-white."

Direction (research/design)

• Carry barrier ingredients on the unified table: barrier_type, gradient_class, passability (CABD status), up_passage_type (fishway), source — instead of (or alongside) the pre-baked blocks_species.
• Classify access late + per-context, per species, reusing fresh's existing label / label_block gradation rather than a fresh binary.
• dam-override: let dams be overridden out of the relevant set by the SAME evidence rules as natural barriers (lnk_barrier_overrides: observations / confirmed habitat / control). Many CABD dams exist on paper but are passable (decommissioned, partial, fishway, or fish demonstrably above). Reuse the rules engine; do NOT build a bespoke fishway model — the name shouldn't bake in the mechanism (fishway is one case).
• This is a deliberate departure from bcfp (bcfp never overrides dams per-species) — opt-in, and it breaks the exact-reproduction correctness bar (CLAUDE.md). Sequence it AFTER the match-bcfp access fix.

Depends on

• mapping_code accessibility: reproduce bcfp barriers_<sp> (natural-only + override) so dam-downstream segments emit ;DAM #200 (access-set reproduction) lands first — it establishes the natural-only per-species access view this generalizes.