math.first_order_intercept_time had a single closing-target test. Add
coverage for zero relative velocity, the a-approx-zero linear fallback, a
target that outruns the projectile (negative determinant), a receding
target with both quadratic roots non-positive, and a closing case with
off-origin, off-axis shooter and target state. The closing cases assert
that the projectile (inheriting the shooter's velocity) and the target
actually coincide at the returned time; the no-solution branches pin the
value the source's own contract specifies.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
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