[BUGFIX] Simplify environment variable check for GEMPY_FLAT_STACKS evaluation

[Leguark]

[BUGFIX] Simplify environment variable check for GEMPY_FLAT_STACKS evaluation

[ENH] Add fault-aware partner filtering for dual contouring surface constraints

• Introduced allowed_partners to restrict pairwise constraint injection based on geological relations.
• Added _build_allowed_partners to compute partner sets using surface_to_stack and faults_relations.
• Updated find_and_inject_multi_surface_constraints_multicore to leverage filtered partners for efficiency

[ENH] Improve fault-layer overlap handling in dual contouring workflow

• Updated _average_overlapping_vertices to handle fault-layer overlaps by copying unperturbed fault vertices to destination layers.
• Refined logic to exclude fault-layer pairs during averaging and QEF constraint injection for improved fault plane smoothness.
• Enhanced tests to verify vertex consistency, excluding fault-destination cases.

[ENH] Add QEF system documentation for dual contouring

• Documented the Quadratic Error Function (QEF) system for standard and multi-surface constraints.
• Included explanations for weighted QEF handling of faults and watertight mesh generation.
• Detailed overlap resolution logic, mesh modifications, and fault

[ENH] Remove fault-pair exclusion logic in QEF setup

• Allowed QEF constraints for fault-related pairs by removing their exclusion during partner filtering.
• Improved handling of overlapping pairs to support fault-aware watertight mesh generation.

[ENH] Refine vertex sharing logic for overlap watertightness

• Updated _average_overlapping_vertices to handle specific cases: same-stack exclusion, fault→layer direct copying, and erosion/onlap pair averaging.
• Improved overlap resolution by clarifying fault and same-stack interaction rules.
• Enhanced logic for vertex averaging to ensure watertight meshes in multi-surface overlap scenarios.

[ENH] Exclude fault-fault pairs and refine partner filtering logic

• Updated fault handling in overlap resolution to exclude fault-fault pairs from QEF injection and triangle sharing.
• Refined partner filtering logic using stack relations to improve fault-aware constraint application.
• Enhanced support for independent fault stacks in dual contouring workflows.

[Leguark]

• [ENH] Optimize kernel data stacking with profiling and GPU-first strategies #57
• [BUGFIX] Simplify environment variable check for GEMPY_FLAT_STACKS evaluation #56 👈 (View in Graphite)
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