feat: add covering quiver support

[pbelmans]

Summary

• add finitely supported dimension vectors on the universal abelian covering quiver, with shifts, Euler forms, and induced finite subquivers
• enumerate canonical connected-support candidates deterministically and retain exactly the nonempty torus-fixed components via lifted stability
• expose tangent-weight dimensions only for compatible dimension vectors with a nonempty stable lift, and return deterministic character-multiplicity pairs (including the zero character when present)
• validate covering vertices, lattice ranks, and positive multiplicities, while preserving custom stability denominators under the covering projection
• compare exact slopes in coprimality checks so the stable-locus guard also works for non-normalized stability data
• add worked documentation and precise references to Boos--Franzen, Weist, and King

The implementation is adapted from QuiverGKM.jl by @dh604, with permission. GKM predicates and moment-graph construction remain in the GKM-specific package and are intentionally outside this PR.

CoveringDimVector still uses mutable vector coordinates for compatibility. The documentation warns against mutating dictionary keys; replacing them with an immutable coordinate type is best handled as a separate API change.

Tests

• Pkg.test() (run by the pre-push hook)
• 180 focused covering-quiver checks
• documentation build
• repository formatter