Avoid false shallow-water barriers from depth roundoff

[woensug-choi]

Interpolating an exactly 2 m seabed can produce values such as -1.9999999999999998, causing valid water to become disconnected and yielding short recovery routes or failed onward planning. Apply a 1e-9 m arithmetic tolerance consistently to swept depth checks and shallow/coastal masks while retaining the configured minimum depth, exclusion buffer, and rejection of unknown terrain.

Validation: all 98 routing tests pass, including regressions that accept boundary roundoff and reject a measurable shallow violation or unknown depth. The same resident terrain produces a checked 2.4 km forward route with the correction.