feat: nullcone motives and wall projections

[pbelmans]

Summary

• compute nullcone motives, dimensions, and defect for quiver settings
• compute fibre dimensions for projections to walls using the shared Oscar-independent VGIT layer from refactor: share Oscar-independent VGIT predicates #49
• check flatness and semismallness stratum by stratum
• document and test the new APIs and their validation

This is stacked on #49, which is itself stacked on #33. Retarget it to main after those dependencies merge.

Tests

• julia --project -e 'using Pkg; Pkg.test()'