feynlag 0.3.0

Added

• diagonalize_svd(method="auto"|"symbolic"|"numeric", dps=50): a numeric
  matrix that is complex, floating-point or larger than 2×2 is decomposed with
  mpmath's complex SVD, returning unitary rotations with
  R_L M R_R† = diag(m ≥ 0) (rows by increasing mass, a round-off singular
  value as an exact zero). Generic complex 3×3 Yukawas now give their masses
  and V_CKM = R_uL·R_dL† inside feynlag (FG-6, #31). Exact 2×2 calls are unchanged.
• Rotation.bar(old_bar, new_bar): the bar-leg partner of a fermion rotation,
  with the conjugate matrix R* (identical to R for a real rotation) (#31).
• GlobalU1: a global (ungauged) U(1) whose assign takes one charge per
  flavour for a fermion (e.g. Froggatt–Nielsen Q_L: (3, 2, 0)), checked by
  the new check_global_invariance and declared on a model with
  Model(global_groups=[...]) (check_invariance/validate report
  global:<name>). Not part of the anomaly check (FG-7, #32). Legs sharing a
  symbolic flavour index are charged flavour by flavour, so a flavour-diagonal
  current ψ̄_i Γ ψ_i (a kinetic term, fermion_gauge_current) is neutral even
  with flavour-dependent charges; an out-of-range integer flavour index, or a
  symbolic index shared by fields with different flavour counts, raises a
  ValueError.

Changed

• The SU(N) groups tutorial (#15) and the Particle Decays tutorial (#30) are
  rebuilt in the derive-then-check style: a prediction before each section,
  and an assert behind every claim instead of printed booleans. They add
  basis-independent invariants, real/complex/pseudo-real representations and
  singlet counting (SU(N)), hand-built Dirac-matrix cross-checks of the
  covariant traces (decays), and web-verified reference lists.

Fixed

• diagonalize_svd no longer returns a silently wrong answer on a complex
  matrix: the symbolic route raises on an explicit I. It also no longer
  crashes with TypeError on a real floating-point input (#31).