Replace observations parameter with barrier_overrides from link

[NewGraphEnvironment]

Problem

fresh 0.12.5 (#69) has observation counting, date filters, species grouping, and threshold logic. This is fish passage interpretation — belongs in link, not the network engine.

Proposed Solution

Replace observations parameter with barrier_overrides — a table of (blue_line_key, downstream_route_measure, species_code) prepared by link. During access gating, fresh skips matched barriers. No counting, no thresholds.

link prepares the table via lnk_barrier_overrides() (NewGraphEnvironment/link#25) using fwa_upstream() SQL directly against fwapg — no fresh dependency needed.

Migration

1. Add barrier_overrides parameter to frs_habitat()
2. Deprecate observations parameter
3. Remove observation counting SQL from frs_habitat_classify.R

Versions

• fresh: 0.12.6
• link: 0.0.0.9000

Relates to NewGraphEnvironment/link#25
Relates to #69
Relates to #122