Refine font-edit tool with O(n) curve fitting

[jserv]

This replaces O(n^4) brute-force spline fitting with O(n) least-squares algorithm using chord-length parameterization. This eliminates the exhaustive search through control point combinations, reducing iterations from ~6M to a single linear pass for typical glyphs.

• Use chord-length parameterization for stable parameter assignment
• Solve 2x2 linear system with Cramer's rule
• Add fallback to linear interpolation for degenerate cases

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Summary by cubic

Replaced O(n^4) brute-force spline fitting with an O(n) least-squares approach using chord-length parameterization, cutting typical glyph fitting from ~6M iterations to a single linear pass. Also hardened memory handling and cleanup to make the font editor more stable.

• Refactors

  • Use chord-length parameterization for point t-values.
  • Solve control points via normal equations (2x2) with Cramer's rule.
  • Add linear interpolation fallback for singular/degenerate cases.

• Bug Fixes

  • Add allocation checks and safe cleanup across command/point handling.
  • Make add_pt return bool; propagate failures up the call stack.
  • Replace recursive copy/free with iterative versions to avoid stack issues.
  • Introduce cleanup() and free_char() to release SDL/cairo and undo stack resources.
  • Guard undo push failures with warnings instead of crashing.

[cubic-dev-ai]

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