Add single-qubit quantum system (kata qat6)

[ellisonbg]

Implements the single-qubit quantum system from kata #qat6 — a
quantum-information-framed single (non-composite) qubit.

What's here

• Rust core (crates/uhlenbeck/src/qubit.rs): Qubit (a Namespace
  impl with dual-cased gate accessors — lowercase default + uppercase
  physics alias returning the same interned atom) and qubit_system /
  qubit_system_for(ns) builders, re-exported from lib.rs. Gates:
  Pauli X/Y/Z, H, S/T + Sdag/Tdag, rotations
  Rx/Ry/Rz, phase P; states qubit(b), zero(), one().
• Symbolic action rules written once over a single wildcard bit fire
  on concrete (X|0>=|1>), symbolic (X|a>=|¬a>), and boolean
  (X|a⊕b>=|¬(a⊕b)>) labels. Plus <b'|b>=δ inner products,
  Hermitian/dagger rules, Pauli commutators (canonical order), and
  completeness Σ|b><b|.
• Python (python/uhlenbeck/systems/qubit.py): Qubit(Namespace)
  mirroring the Rust accessors, builds + registers on init (idempotent
  reuse). New uhlenbeck.systems subpackage.
• Notebook (notebooks/systems/qubit.ipynb): tutorial authored +
  executed clean via nb execute --uv.
• Tests: crates/uhlenbeck/tests/qubit.rs (Rust) and
  tests/test_qubit.py (Python).

Design note — boolean ↔ {0,1} coercion

The issue's central design question resolves to no special machinery
needed: boolean true/false are the integers 1/0, so a
concrete-bit label folds automatically inside pow/exp
((−1)^1=−1, i^1=i, exp(iπ·0/4)=1) while a symbolic or
boolean-expression label leaves the phase symbolic ((−1)^a,
(−1)^(a⊕b)). One rule handles all three label kinds.

Verification

• cargo test -p uhlenbeck green (full suite); cargo fmt --all --check
  and clippy clean.
• uv run pytest tests/ — full suite green.
• Notebook executes with no errors (H·H|0>=|0>, HZH|0>=|1>, etc.).

Follow-ups (filed)

• #8agy — Inner-product evaluation API: no Python/evaluate path
  fires inner_product_rules today (IP evaluation is Rust-only via
  eval_inner_product_once).
• #nyxn — Triggering API for commutator_rules/anticommutator_rules
  (declarative data with no user-facing trigger).
• #p7er — Qubit(register=False) can build a malformed system on an
  already-registered namespace (build-before-register dagger hazard).

🤖 Generated with Claude Code