feat: Domokos reduction for quiver moduli

[pbelmans]

Implements the two reduction operations from Domokos, Quiver moduli spaces of a given dimension (doi:10.4171/JCA/97):

• tau_reduction (τ_u) merges and deletes a large vertex (Definition 2.1, Lemma 3.1);
• sigma_reduction (σ_u) reflects a small source or sink (Definition 2.2, Lemma 3.3).

Both operations have pair-only (Q, d) methods matching the paper and weight-aware (Q, d, theta) methods that transport a King-normalized stability parameter. QuiverModuliSpace overloads return an isomorphic moduli problem with the standard denominator sum, rather than reusing a closure tied to the original vertex set.

The PR also provides is_large, is_small_source, is_small_sink, the terminating greedy driver tau_sigma_reduce, and is_taus_minimal for Definition 2.3 in the class of all sincere quiver-dimension-vector pairs.

Correctness and contracts

• The branch is rebased onto the corrected bocklandt head.
• Sincerity is asserted throughout the Domokos API.
• Weight-aware entry points validate vector lengths, King normalization, and existence of semistable representations; output normalization is asserted.
• is_taus_minimal depends only on (Q, d), as the cited definition does.
• A state limit throws an indeterminate-search error instead of returning true; true now means the reachable reduction graph was exhausted.
• Reduction arithmetic is checked, so a growing reflection orbit raises OverflowError instead of wrapping into a false witness.
• The breadth-first queue uses a head index rather than repeated popfirst! calls.

Paper example and search limits

The paper does not give a finite bound for deciding Definition 2.3 by graph search. In Section 9 it proves its infinite-orbit example minimal by a case split and inequalities that force every reflection sequence to keep increasing total dimension. The docstring now explains this and includes both that example and a four-vertex pair where a cap of one is indeterminate but a cap of 100 finds a delayed tau reduction.

Tests

The full local suite passes, including doctests and 31 Domokos tests. The regressions cover:

• pair-only and weight-aware tau and sigma reductions;
• preservation of Hodge diamonds and dimension in the cited examples;
• sigma involutivity and the greedy fixpoint;
• sincerity, normalization, and semistability validation;
• replacement of custom denominator closures;
• bounded-search and delayed-reduction behavior;
• checked overflow along the Section 9 orbit.