feat: framed quiver moduli spaces, local structure, Proposition 6 fibres

[pbelmans]

Stacked on top of #33 (bocklandt). Adds framed quiver moduli spaces and the fibres of the
framed projection (Proposition 6 of arXiv:2607.12895,
https://arxiv.org/abs/2607.12895), together with the etale-local structure of Luna strata
(Proposition 5), and makes local_quiver_setting public (closes #41).

Projection-to-the-wall fibres (Proposition 8) are intentionally left to #33, which already
implements them functionally (fibre_dimension, is_flat, is_semismall).

What is added

Quiver-level framing (parity with the Python/Sage QuiverTools):

• framed_quiver(Q, n): prepends a framing vertex with n[i] arrows i0 -> i.
• coframed_quiver(Q, n): appends a coframing vertex with n[i] arrows i -> i0.

A framed quiver moduli space, encoded as a new type FramedQuiverModuliSpace storing the
base (Q, d, theta) and the framing datum n:

• base(X): the base moduli space M^theta(Q, d), the codomain of the projection p.
• total_space(X): X itself as an ordinary QuiverModuliSpace on the framed quiver.
• framing_vector(X): the framing datum n.
• fibre(X, tau): Proposition 6. The fibre of p over the Luna stratum S_tau, as a
  NilpotentLocus inside a framed moduli of the local quiver, with local framing datum
  n_tau = sum_k (n . d_k) i_k (using the summands key that feat: Bocklandt reduction and smoothness of quiver moduli #33 adds to
  local_quiver_setting).

NilpotentLocus(ambient) marks the nilpotent sublocus of a moduli space; ambient(N)
recovers the ambient. local_structure(M, tau) returns the etale-local model
M^0(Q_tau, d_tau) of the singularity along S_tau (Proposition 5). local_quiver_setting
is now exported (closes #41).

API note (deliberate deviation)

FramedQuiverModuliSpace and NilpotentLocus introduce an object-oriented flavour that
deviates from the current function-based API (e.g. local_quiver_setting returns a Dict).
Consequently fibre returns an object rather than a data dictionary. This was a deliberate
choice: a framed quiver moduli space carries data (base plus framing) that a plain
QuiverModuliSpace on the framed quiver would discard, and Proposition 6 states the fibre as
itself a framed moduli space.

Nilpotency caveat

The fibre is the nilpotent locus inside total_space(ambient(fibre(X, tau))); that
nilpotency is recorded, not computed. (#33's dimension_nullcone / nullcone_motive are the
natural way to compute it, which a follow-up can wire into NilpotentLocus.)

Testing

• Reproduces the QM_1 example of arXiv:2607.12895: the framed affine-D4 quiver is a
  fourfold, and its five fibre types (Lemmas 18-22) have the local dimension vectors and
  local framing data stated in the paper.
• Quiver-level framing/coframing adjacency, the accessors, local_structure, and the public
  local_quiver_setting.
• Full test suite (including feat: Bocklandt reduction and smoothness of quiver moduli #33's testsets and the doctests) passes.