Fix precision validation gaps and enhance insert() capabilities

[atksh]

This commit addresses multiple precision and validation issues identified
in the codebase analysis:

1. Input Validation

• Add NaN/Inf validation to insert() methods for both float32 and float64
• Ensures consistency with constructor validation
• Prevents invalid data from entering the tree structure

2. Float64 Insert Support

• Add float64 overload for insert() method
• Maintains idx2exact map for dynamically inserted items
• Preserves double-precision refinement capability for inserted boxes
• Uses explicit py::overload_cast in Python bindings to handle overloads

3. Precision Testing

• Add comprehensive tests for NaN/Inf validation in insert operations
• Add tests for float64 insert() maintaining precision
• Add tests verifying rebuild() preserves idx2exact
• Add systematic precision boundary tests (adjusted for float32 limits)
• Document float32 precision limitations in test comments

Technical Notes

• Float64 input is converted to float32 for tree structure
• Double-precision refinement helps reduce false positives
• Precision limits: gaps below ~1e-7 may not be reliably detected
• At large magnitudes (e.g., 1e6), absolute precision degrades

Fixes validation gaps in insert operations and maintains precision
capabilities for dynamically updated trees.