Simulator optimization branch: GPU density matrix, perf, audits, shared context

Major work on branch (squash of 28 commits + final clippy cleanup):

GPU simulator optimizations
- f32 gate fusion, commuting-gate reorder, CY/SWAP/RXX/RYY direct shaders
- Persistent kernel for small states with dynamic shared-memory sizing
- CPU measurement fast path for small states (f32 <=16q, f64 <=15q)
- Parallel CX/CZ/RZZ/RXX/RYY via rayon when .parallel(true)
- Parallel scalar CX path for low-qubit pairs (q_lo < 2)
- Fused flush_gates + state() readback into single encoder
- Adaptive mz path selection: empirical N/M lookup table replacing hardcoded threshold
- Raised StateVecSoA parallel threshold 14 -> 21 qubits
- Exploration benchmarks for adaptive path decisions

GpuDensityMatrix
- Choi-Jamiolkowski representation on top of GpuStateVec
- Generic over backend (f32 / f64); gates, noise channels, helpers
- Cholesky re-purification for mixed states

Correctness fixes (from audits)
- GPU 2q rotation shaders (RXX/RYY bit_a==bit_b bug)
- DensityMatrix phase/amplitude damping trace preservation
- mz is_deterministic flag (previously hardcoded false)
- GpuPauliProp gate ordering + Pauli X/Y/Z semantics, stale buffer reads
- GpuDensityMatrix mz probability formula
- GpuStabMulti::mz_queue now snapshots state at call time

Shared GPU context (concurrent-use SIGSEGV fix)
- Process-wide OnceLock<GpuDeviceContext> in pecos-gpu-sims/src/gpu_probe.rs
- All 7 simulators now reuse one wgpu Instance/Adapter/Device/Queue
- Fixes crashes under cargo's parallel test harness and MonteCarloEngine shots
- Removed the --test-threads=1 workaround from pecos-cli rust test

Test infrastructure
- New audits: gate_audit, gate_fuzz, pauli_prop_audit, influence_sampler_audit,
  large_n_audit, noisy_sampler_stats, stab_extra_audits, extra_audits,
  flush_blocked_audit, concurrent_gpu_test
- Removed pecos-quest / pecos-qulacs wrapper crates (bench code only)
- Removed dangling quest_sim_test.rs and quest_example.rs

Clippy / lint cleanup
- GpuError::Startup variant wraps GpuStartupError
- Internal GPU constants now usize (casts removed)
- Renamed GatePipeline variants SWAP/RXX/RYY/RZZ -> Swap/Rxx/Ryy/Rzz
- # Panics / # Errors doc sections added where clippy required
- Cholesky loops allow needless_range_loop with justification
- Test files allow cast_possible_truncation / cast_precision_loss with rationale

Author: ciaranra

Cargo.lock

@@ -192,8 +192,6 @@ pecos-qis = { version = "0.2.0-dev.0", path = "crates/pecos-qis" }
 pecos-qis-ffi = { version = "0.2.0-dev.0", path = "crates/pecos-qis-ffi" }
 pecos-qis-ffi-types = { version = "0.2.0-dev.0", path = "crates/pecos-qis-ffi-types" }
 pecos-quantum = { version = "0.2.0-dev.0", path = "crates/pecos-quantum" }
-pecos-quest = { version = "0.2.0-dev.0", path = "crates/pecos-quest" }
-pecos-qulacs = { version = "0.2.0-dev.0", path = "crates/pecos-qulacs" }
 pecos-random = { version = "0.2.0-dev.0", path = "crates/pecos-random" }
 pecos-relay-bp = { version = "0.2.0-dev.0", path = "crates/pecos-relay-bp" }
 pecos-rslib = { version = "0.2.0-dev.0", path = "python/pecos-rslib" }

Cargo.toml

@@ -17,17 +17,13 @@ default = []
 parallel = ["pecos-simulators/parallel"]
 gpu-sims = ["dep:pecos-gpu-sims"]
 cuquantum = ["dep:pecos-cuquantum"]
-quest = ["dep:pecos-quest"]
-qulacs = ["dep:pecos-qulacs"]
 cppsparsestab = ["dep:pecos-cppsparsestab"]
-all-sims = ["gpu-sims", "cuquantum", "quest", "qulacs", "cppsparsestab"]
+all-sims = ["gpu-sims", "cuquantum", "cppsparsestab"]
 
 [dependencies]
 # Optional simulator dependencies for benchmarking
 pecos-gpu-sims = { workspace = true, optional = true }
 pecos-cuquantum = { workspace = true, optional = true }
-pecos-quest = { workspace = true, optional = true }
-pecos-qulacs = { workspace = true, optional = true }
 pecos-cppsparsestab = { workspace = true, optional = true }
 pecos-core.workspace = true
 pecos-simulators.workspace = true