Add boundary-aware overloads for canonicalize_for_rounds and sliding window decoder

[eliotheinrich]

Description

Closes #649

The addition of boundary detectors offsets the syndromes output by memory circuit experiments by introducing num_boundary_syndromes at the beginning and end of the syndrome block. The means that canonicalize_for_rounds straddles the new boundaries between rounds, leading to error mechanisms being associated to incorrect rounds when the boundary syndromes have a different length than the inter-round syndromes. This is a cosmetic error for PCMs representing entire memory circuits, but is incorrect when followed up with get_pcm_for_rounds for the purpose of, i.e., the sliding window decoder.

This PR adds round-aware overloads to canonicalize_for_rounds and the sliding window decoder, bringing them up-to-date with syndrome format.

canonicalize_for_rounds has a new overload which accepts num_boundary_syndromes, which appropriately treats the first and last num_boundary_syndromes columns as distinct rounds for the purpose of round-based topological sorting.

The sliding window decoder now takes an additional parameter num_boundary_syndromes (default 0). The PCM of the decoder is zero-padded to have uniform width num_syndromes_per_round. Syndromes passed to the decoder are automatically padded to the uniform length. If num_boundary_syndromes is 0, then the boundary syndromes are presumed to be the same width as the interior syndromes (as is the case in X/Z-only experiments).

New tests:

test_qec.cpp:PCMUtilsTester.checkGetSortedColumnIndicesWithBoundary: Verifies the boundary-aware column sort orders
test_qec.cpp:DetectorErrorModelTest.CanonicalizeBoundaryRejectsWideBoundary: Assertrs canonicalize_for_rounds(S, B, ...) throws when the boundary is wider than the interior (B > S).
test_decoders.cpp::SlidingWindowDecoder.PreparePcmRejectsBadBoundaryLayout: Confirms the sliding-window decoder throws during construction for inconsistent boundary layouts (B > S, or a row count that can't fit [B | K·S | B]).
test_qec_stim.cpp::QECCodeTester.checkDemFromMemoryCircuitShor9: Checks the Shor [[9,1,3]] full DEM has the expected non-uniform shape, consistent column counts, and correct round ordering.
test_qec_stim.cpp::QECCodeTester.checkSlidingWindowShor9Boundary: A single all-rounds window decoding the Shor boundary DEM must match a full decoder for every DEM column.
test_qec_stim.cpp:QECCodeTester.checkSlidingWindowShor9Streaming: Streams variable-width detector layers for X(Z) (6(2) at boundaries, 8 interior) through a sliding config (window=2, step=1) and requires results to match a full decoder.
test_sliding_window.py::test_sliding_window_boundary_layout: Parametrized over 3 codes × 3 window counts; injects errors over 300 shots and asserts sliding-window observable predictions match a single_error_lut full decoder.

Runtime / performance impact

N/A

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[bmhowe23]

Can you please move this section to a .cpp file? I don't think it needs to be in a public header file if we are calling it an "internal helper".

[eliotheinrich]

I can, but I deliberately left it accessible here so that the round layout logic can be shared here and in the sliding window decoder (and perhaps other places that may need a mapping between rounds and detector indices): https://github.com/NVIDIA/cudaqx/pull/656/changes#diff-30af01af4513a257ae4040df674193bc2cce9d3ce09e81adc3493db11a0dba4dR60.

Open to moving it to a .cpp file if we'd rather not expose it.