feat: Bocklandt reduction and smoothness of quiver moduli

[pbelmans]

Implements the reduction algorithm of Bocklandt (MR1929191) and the classification of cofree quiver settings by Bocklandt and Van de Weyer (doi:10.1016/j.jalgebra.2007.08.019), and uses them for the geometry of moduli spaces:

• strongly_connected_components for quivers,
• bocklandt_reduction and is_coregular for quiver settings, with every reduction step documented against the paper,
• is_smooth for moduli spaces with properly semistable representations, by checking coregularity of the local quiver setting of every Luna type (MR1972892),
• codimension_singular_locus for moduli spaces, as the singular locus is a union of Luna strata,
• is_cofree for quiver settings, via wedging, prime components and the classification lists,
• nullcone_motive, dimension_nullcone and defect: the motive of the nullcone is a polynomial in the Lefschetz motive, computed by the socle recursion of Gösmann--Reineke (doi:10.3842/SIGMA.2026.020); cofree = coregular + zero defect (Popov) is verified on 100 settings in the test suite,
• fibre_dimension, is_flat and is_semismall for projections to walls, computed through nilpotent semistable moduli of local quiver settings; the source stability parameter is required to be King-normalized.

Tests include the ten nodes of the Segre cubic, the accidental isomorphism to Gr(2, 4) for the 6-subspace quiver on a wall together with its fibre dimensions and semismallness, and P^5 for the 3-Kronecker quiver with d = (2, 2) and d = (2, 4).

[pbelmans]

@giannipetrella This is a Claude-assisted implementation of Raf's smoothness check. Let's review this carefully, and then we have another very useful feature ready.