Add triangular color code plugin with superdense memory circuit

[kvmto]

Description

Adds the triangular color code to CUDA-QX as a Python QEC code plugin, together with a superdense memory circuit built directly on CUDA-Q's detector annotations.

color_code plugin (plugins/codes/color_code.py):

• ColorCodeGeometry: triangular color code with a rectangular-grid embedding (data-qubit grid, algorithmic plaquette generation, syndrome-to-grid mapping, physical circuit layout, superdense plaquette labeling), ported from an internal reference implementation.
• CSS stabilizers/observables and patch-based prep0/prep1/prepp/prepm + stabilizer_round kernels, registered via @qec.code('color_code') (required option: distance, odd ≥ 3). Parity/observable matrices are provided through the standard code API (get_parity_x/z, get_observables_x/z).

Superdense memory circuit (same module):

• The paired-ancilla, depth-8 CNOT schedule measures both stabilizer bases of every plaquette each round. The measurement feedforward of the superdense readout is absorbed analytically into explicit cudaq.detector / cudaq.logical_observable annotations, so the circuit is straight-line Clifford and cudaq.dem_from_kernel accepts it (stim's detector-determinism analysis passes, d ≤ 11 checked).
• SuperdensePlan exposes the detector record layout and per-detector coordinates (including the color annotation scheme used by color-code decoders such as Chromobius) for upcoming decoder integration; superdense_dem / superdense_sample provide DEM extraction and detector-ordered sampling.

Validation

• Geometry pinned against reference plaquette tables (d = 3/5/7) and structural invariants up to d = 13.
• Detector/observable record sets pinned against literals extracted from the reference construction.
• Golden-oracle test: the DEM of the CUDA-Q kernel is equivalent to the reference stim construction of the same circuit under matched noise — identical error mechanisms and probabilities, both memory bases (golden files under tests/data/).
• Decoding sanity check (single_error_lut) shows the expected logical-error reduction.
• 98 new test items, ~5 s wall clock on the stim target.

Notes for reviewers

• ColorCodeGeometry.superdense_plaquette contains an intentional unreachable return kept for consistency with the reference implementation.
• Framework-path E2E tests (qec.sample_memory_circuit on this plugin) are prepared but intentionally not included: Python-defined codes currently hit test_code.py::test_python_code disabled by #636 #646 (root cause upstream, fix validated locally against cuda-quantum#4861). They will follow once the CUDA-Q pin includes that fix.
• Open question: whether the superdense functionality should stay in plugins/codes/ or move elsewhere / become framework-level API once test_code.py::test_python_code disabled by #636 #646 is resolved.

Status

Draft: decoder (Chromobius) integration on top of SuperdensePlan.detector_coords is planned as the next step.