Fix misleading comment on water_cells in dig_rivers.py

[Copilot]

The comment preceding the water_cells computation referred to checking "open sides" — logic that was already removed — leaving a stale, inaccurate description that would confuse future maintainers.

Changes

• src/bathytools/actions/dig_rivers.py: Replaced the outdated # Check if a side is "open" comment with an accurate description of what water_cells is and where it's actually used:

# Precompute a boolean array of cells that are already underwater
# (negative bathymetry). Used later to skip water cells when tracking
# river dig cells for intersection detection.
b_array = bathymetry.elevation.transpose("latitude", "longitude").values
water_cells = b_array < 0

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[Copilot]

Pull request overview

Updates an outdated inline comment in dig_rivers.py so it accurately describes the water_cells precomputation and its later role in skipping already-underwater cells during river intersection tracking.

Changes:

• Replaced a stale “open side” comment with an accurate description of water_cells (underwater/negative bathymetry).
• Clarified why the boolean mask is computed (used later when recording dig cells for intersection detection).

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