Add a QuantumCumulants.jl extension for express

[AJ0070]

Closes #73.

Adds a QuantumCumulants.jl extension, so symbolic operators can be converted into the
second-quantized operators QuantumCumulants works with and fed to meanfield,
cumulant_expansion, etc.

h = FockSpace(:cavity) ⊗ NLevelSpace(:atom, (:g,:e))
repr = QuantumCumulantsRepr(h, [:a, :σ])

H = express(Δ*(n̂ ⊗ Iₐ) + g*(âꜛ ⊗ σ₋ + â ⊗ σ₊), repr)   # Δ*a'*a + g*(a'*σ₁₂ + a*σ₂₁)
J = [express(â ⊗ Iₐ, repr), express(Ic ⊗ σ₋, repr)]
eqs = complete(cumulant_expansion(meanfield([a'*a, σ(:e,:e)], H, J; rates=[κ, γ]), 2))

Changes

• ext/QuantumCumulantsExt/QuantumCumulantsExt.jl (+251) — the conversion: Fock operators,
  qubit operators as Transitions on an NLevelSpace or Paulis on a PauliSpace,
  projectors/outer products (incl. qudits), identities, tensor products placed across
  subsystems, sums/products/scalings/commutators/daggers.
• src/QSymbolicsBase/QSymbolicsBase.jl (+45 −1) — exports QuantumCumulantsRepr. The
  Hilbert space is given explicitly or derived from the expression's basis.
• test/general/quantumcumulants_tests.jl (+200) — 75 tests.
• docs/src/quantumcumulants.md (+123), docs/make.jl, docs/src/express.md — new docs page.
• Project.toml + test/docs/benchmark manifests, CHANGELOG.md — weakdep, compat 0.7,
  version bump to 0.4.17.

Notes

• Z₁ = |0⟩ maps to the excited level, so σ₋ becomes the decay operator σ^ge.
• Both packages export Destroy, Create, dagger, commutator; the clash is documented,
  with the unicode aliases (â, âꜛ, n̂) as the escape hatch.
• Unitaries, two-qubit gates, MixedState, conj/transpose and partial traces throw —
  none are expressible in a polynomial operator algebra.

AI Disclosure: I used Claude Opus 5 to initially understand the problem, code scaffolding, adding comments, documentation, and getting full test coverage.

[codecov]

Codecov Report

❌ Patch coverage is 87.28814% with 15 lines in your changes missing coverage. Please review.
✅ Project coverage is 71.11%. Comparing base (35154cf) to head (229648b).

Files with missing lines	Patch %	Lines
ext/QuantumCumulantsExt/QuantumCumulantsExt.jl	87.38%	14 Missing ⚠️
src/QSymbolicsBase/QSymbolicsBase.jl	85.71%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #216      +/-   ##
==========================================
+ Coverage   68.89%   71.11%   +2.22%     
==========================================
  Files          21       22       +1     
  Lines         987     1101     +114     
==========================================
+ Hits          680      783     +103     
- Misses        307      318      +11     

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[AJ0070]

@Krastanov gentle ping!

[AJ0070]

@Krastanov