refactor: replace RatComplexNum by Mathlib's GaussianInt

[zhikaip]

1. Switch to GaussianInt, rename ofRat → ofGaussianInt

Every coefficient that went through Physlib.RatComplexNum is 0, ±1, ±i or an integer
Levi-Civita sign, so Mathlib's GaussianInt (GaussianInt.toComplex,
GaussianInt.toComplex_injective) replaces it. No statement about Pauli matrices,
metrics, units or the Levi-Civita tensor changes.

• Removed Physlib/Mathematics/RatComplexNum.lean (the type, its ring structure,
  toComplexNum and its helper lemmas).
• Renamed ComplexTensor/OfRat.lean → OfGaussianInt.lean, and every …ofRat…
  declaration → …ofGaussianInt… (33 names); contr_basis_ratComplexNum →
  contr_basis_gaussianInt.
• pauliContrComponent, pauliContrDownComponent and the Levi-Civita components are
  now GaussianInt-valued.

2. Mini-refactor: define ofGaussianInt via Tensor.ofComponents

ofGaussianInt f is now ofComponents c (fun b => GaussianInt.toComplex (f b)), reusing
the existing tensor API instead of going through (Tensor.basis c).repr.symm and
Finsupp.linearEquivFunOnFinite. ofGaussianInt_basis_repr_apply now follows from
componentMap_ofComponents. Performance-neutral.

Performance (heartbeats, same base)

File	master	this PR
PauliMatrices/Relations.lean	770,444k	707,080k (−8.2%)
PauliMatrices/ToTensor.lean (declarations > 20M)	538,614k	521,041k (−3.3%)

The gain comes from commit 1: kernel decide is cheaper on integer than on rational
arithmetic. No declaration got slower.

Reviewer map

Review commit by commit. In commit 1, start with ComplexTensor/OfGaussianInt.lean,
then PauliMatrices/ToTensor.lean, LeviCivita/Complex.lean and
PauliMatrices/Relations.lean; the rest is the mechanical rename. Commit 2 touches only
OfGaussianInt.lean.

Disclaimer: I instructed claude to make these changes and everything looks reasonable to me

🤖 Generated with Claude Code

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[jstoobysmith]

Approved - many thanks. Looks good to me.