frs_habitat: watershed group habitat pipeline

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Problem

No way to run the fresh habitat pipeline (extract, break, classify, categorize) across all species in a watershed group. The vignette demonstrates the pattern for one species on a subbasin. Scaling to a full WSG with all species needs orchestration.

Baseline timing (BULK, 32472 segments, 7 species)

Species	Total	Access	Classify	Categorize
BT	162s	120s	34s	1s
CH	180s	130s	43s	1s
CO	207s	163s	36s	1s
PK	173s	135s	32s	1s
RB	163s	121s	35s	2s
SK	167s	126s	33s	1s
ST	155s	111s	37s	1s

Total: 20 min. Access barriers (frs_break_find gradient mode) = 70-80% of cost.

Key optimization: deduplicate access barriers

Species share access_gradient_max thresholds:

• 0.15: CH, CO, PK, SK (4 species, 1 gradient scan)
• 0.25: BT, RB, CT, DV (4 species, 1 gradient scan)
• 0.20: ST (1 species, 1 gradient scan)

Falls barriers are species-independent. Computing access once per unique threshold instead of per species cuts the bottleneck from 7 scans to 3. Estimated 8-10 min for BULK.

Proposed functions

• frs_access(conn, table, threshold, to) - gradient breaks + falls at one threshold
• frs_species(conn, species_code, base_tbl, breaks_tbl, params) - copy base, apply pre-computed breaks, classify, categorize
• frs_habitat(conn, wsg) - orchestrator: extract base, group species by threshold, fan out, cleanup

Output tables mirror bcfishpass: working.streams_co, working.streams_bt, etc. Per-species tables because break points create different segment geometries per threshold group.

Parallelism (furrr)

After access barriers are pre-computed, per-species work is about 40s (copy + classify + categorize). These are independent (separate output tables), parallelizable with furrr across species within a WSG, or across WSGs for multi-group runs.

Supporting pieces (already on feature/extract-where)

• frs_wsg_species() - lookup species and bcfishpass view names per WSG from bundled wsg_species_presence.csv
• frs_extract(where=) - column filter for fast WSG extraction (Add where parameter to frs_extract() #60)

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frs_habitat() orchestrator shipped in v0.8.0. Commit 993d720.