Add spawn/rear gradient min to habitat thresholds CSV

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Problem

bcfishpass parameters_habitat_thresholds.csv has spawn_gradient_max but no spawn_gradient_min — implicitly classifies gradient 0 to max as spawning habitat. Salmonids do not spawn in flat water — a minimum gradient threshold is needed to exclude low-gradient reaches (pools, lake outlets, slack water).

Proposed Solution

Add spawn_gradient_min and rear_gradient_min columns to inst/extdata/parameters_habitat_thresholds.csv.

Defaults: spawn_gradient_min = 0.005 (0.5%) for all species, rear_gradient_min = 0 for all species.

Update .frs_build_ranges() in R/frs_params.R to read the min from the CSV instead of hardcoding 0.

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spawn_gradient_min added to parameters_fresh.csv. Commit b981b36.