Keep multiple labels per position in streams_breaks (no barrier identity loss)

[NewGraphEnvironment]

Problem

streams_breaks deduplicates breaks at shared positions — when a gradient barrier and a crossing land at the same measure, one label survives. Barrier identity is lost. This breaks:

1. barrier_overrides — can't match a gradient barrier that was merged with a crossing
2. Access reporting — can't distinguish barrier types downstream of a segment
3. Habitat reporting — can't report km behind falls vs gradient vs crossings separately

bcfishpass v0.5.0 keeps separate barrier tables that persist independently. Each segment knows which barriers of each type are downstream via separate columns (barriers_bt_dnstr, barriers_falls_dnstr, etc.).

Proposed Solution

Allow multiple labels per position in streams_breaks. Change the unique constraint from (blue_line_key, downstream_route_measure) to (blue_line_key, downstream_route_measure, label).

A position can be both a crossing AND a gradient_15 barrier:

blue_line_key | downstream_route_measure | label
356362743     | 196606                   | potential      (crossing)
356362743     | 196606                   | gradient_15    (gradient barrier)

Segmentation still deduplicates for geometry splitting (one cut per position). But the breaks table retains all labels. The access model queries by label. Overrides match by label + position.

This enables per-segment barrier composition: "this segment is behind gradient_15 + falls + crossing_potential" — each independently queryable, overridable, reportable.

Impact on existing code

• frs_network_segment: change dedup logic — deduplicate geometry cuts but not label entries
• Access SQL: no change — already filters by label
• barrier_overrides: works as designed — matches label + position
• Clustering: no change — uses labels already

Versions

• fresh: 0.13.1
• bcfishpass: v0.5.0
• link: 0.1.0

Relates to NewGraphEnvironment/link#16
Relates to NewGraphEnvironment/link#25
Relates to #143

[NewGraphEnvironment]

Implementation detail

Where dedup happens

Search for ON CONFLICT or DISTINCT ON or GROUP BY blue_line_key, downstream_route_measure in frs_network_segment.R or the SQL it generates. That is where labels get dropped when two break sources land at the same position.

Composite key

Use (blue_line_key, downstream_route_measure, label) as the unique constraint. Preserve source column per label — it tracks provenance of each break.

Access SQL interaction

Current access SQL in frs_habitat_classify.R uses NOT EXISTS against streams_breaks filtered by label. This still works with multiple rows per position — NOT EXISTS returns FALSE if ANY matching label exists.

barrier_overrides interaction

barrier_overrides has (blue_line_key, downstream_route_measure, species_code). An override on a gradient_15 at position X should NOT remove a falls at the same position. The override targets gradient labels only. The access SQL already filters by label pattern so this should work — overriding gradient_15 still leaves the falls label blocking at that position.

Test

ADMS comparison (link 0.1.0). After the fix:

• lnk_barrier_overrides should find ~95 overrides from streams_breaks (currently 35 due to label loss)
• BT accessible should increase from 6,756 to ~8,700
• BT/CH/CO should return to within 5% of bcfishpass v0.5.0

Versions

• fresh: 0.13.1, bcfishpass: v0.5.0, link: 0.1.0