Add Boson and Fermion quantum systems (kata #sqdq, #cqv1)

[ellisonbg]

Adds two built-in production quantum systems, mirrored across the Rust core and the Python package, implementing kata issues #sqdq (Boson) and #cqv1 (Fermion) under parent #5yzk.

Both follow the established two-layer pattern: a pure-data QuantumSystem builder plus a Namespace-implementing handle.

Boson — abstract single bosonic mode (#sqdq)

The ladder-operator algebra a/a†/N on a bosonic Fock space — not the harmonic oscillator (no x/p).

• Action: a|n⟩=√n|n−1⟩ (incl. a|0⟩=0), a†|n⟩=√(n+1)|n+1⟩, N|n⟩=n|n⟩, plus bra-side rules.
• ⟨m|n⟩=δ, dagger (a↔a†, N Hermitian), completeness Σ|n⟩⟨n|.
• [a,a†]=1, [N,a]=−a, [N,a†]=a† in the commutator table; ByHead([a†, a]) policy.

Fermion — single fermionic mode (#cqv1)

Occupation basis c/c†/N, states |0⟩/|1⟩, mirroring sympy physics.quantum.fermion.

• Pauli action: c|0⟩=0, c|1⟩=|0⟩, c†|0⟩=|1⟩, c†|1⟩=0, N|k⟩=k|k⟩.
• ⟨i|j⟩=δ, dagger, completeness Σ_{k=0}^1|k⟩⟨k|.
• Anticommutators {c,c†}=1, {c,c}=0, {c†,c†}=0 stored in the algebra table as data ({c,c} stays a symbolic Anticommutator).

What's where

• Rust: crates/uhlenbeck/src/systems/{mod,boson,fermion}.rs, re-exported as uhlenbeck::{Boson, Fermion}; tests in tests/systems_{boson,fermion}.rs.
• Python: uhlenbeck.systems.{Boson,Fermion} (build + register on init, with notebook-re-run safety), also top-level uhlenbeck.{Boson,Fermion}; tests in tests/test_systems_{boson,fermion}.py.
• Notebooks: notebooks/systems/{bosons,fermions}.ipynb (executed).

Notes / scope

• Per qapply spec §1.2, qapply leaves ⟨m|n⟩ symbolic; the matrix element ⟨2|a†a|2⟩ reduces to 2·⟨2|2⟩ and inner-product collapse is tested via eval_inner_product_once.
• The Boson docs were corrected (2nd commit) to state that normal_order does consume the bosonic commutator (a·a†→a†·a+1), while the fermionic anticommutator is not yet consumed.
• Follow-ups tracked as kata issues, intentionally not in this PR:
  • #g13f — derive normal_order swaps from anticommutators (fermion support).
  • #3zrd — Rust dagger-rule construction is unsafe when rebuilding an already-registered namespace.

Verification

cargo fmt --all --check clean, cargo clippy --workspace --tests clean, full Rust workspace tests pass, Python suite 819 passed, both notebooks execute end-to-end.

🤖 Generated with Claude Code