App beta backprop

[bogsdarking]

Added

• Concavity penalty for the L2 annotation cost function. The optimizer can occasionally
  converge to a physically wrong, inverted arc — a failure mode on off-apex images or with
  loose parameter bounds. The penalty detects this by checking whether the predicted arc
  curves in the correct direction relative to its chord, and adds a large cost when it does
  not. Enabled by default; configurable via concavity_penalty (bool) and
  concavity_penalty_scale (float) kwargs in fit_arc(). The correct curvature direction
  is auto-detected from theta_z, so non-standard viewing geometries (e.g. camera pointing
  away from nadir) are handled correctly without any user configuration.

Changed

• Sagitta pre-fitter (fit_sagitta) upgraded from a 2-D to a 3-D optimisation: the arc
  apex x-position is now a free parameter alongside curvature and camera angle. This removes
  a systematic radius bias on asymmetric arcs whose true apex falls outside the annotated
  region.
• Sagitta pre-fitter now seeds the differential-evolution warm-start with a geometrically
  computed camera angle (arccos(r / (r + h))) rather than the raw optimiser estimate,
  avoiding a near-degeneracy that could misguide the final fit.

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Codecov Report

❌ Patch coverage is 88.67925% with 6 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
planet_ruler/fit.py	88.67%	4 Missing and 2 partials ⚠️

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