Sparse-aware PCM utility migration

[vedika-saravanan]

Description

Adds sparse-aware paths for the round-agnostic PCM utilities:

• reorder_pcm_columns
• shuffle_pcm_columns
• pcm_to_sparse_vec
• pcm_to_sparse_string

The C++ API now has sparse_binary_matrix overloads for those utilities. The Python bindings also accept scipy sparse inputs for the Python-exposed functions (reorder_pcm_columns, shuffle_pcm_columns, and pcm_to_sparse_vec) without materializing a dense cudaqx::tensor.

This intentionally avoids utilities that depend on a fixed scalar num_syndromes_per_round, since those APIs are expected to change for variable syndrome counts per round.

Related: #582

Runtime / performance impact

Avoids dense cudaqx::tensor<uint8_t> allocation for the covered sparse utility paths when callers provide sparse_binary_matrix in C++ or scipy sparse matrices in Python.

Self-review checklist

Please confirm each item before requesting review. Check [x] or strike
through and explain.

Before requesting review

• I reviewed my own full diff in GitHub or my editor.
• PR is in Draft if it is not yet ready for review.
• Temporary / debugging changes have been removed.
• Local test logs reviewed; no unexplained warnings or errors.
  Local C++ tests not run: cmake is unavailable in this shell.
• CI logs reviewed; no unexplained warnings or errors.
  Pending CI.
• Full CI has been run.
  Pending CI.

Scope and size

• PR is under ~1000 lines, or an exception is justified in the description.
• Refactoring-only changes are isolated in their own PR(s).
• No existing tests were disabled or modified just to make this PR pass
  (if so, an issue has been raised).

Tests

• New functionality has new tests.
• Tests fail if the new functionality is broken (including crashes), not
  just when it is missing.
• Negative tests added where exceptions are expected.
  N/A: the new sparse utility paths are covered by positive/equivalence tests; no new expected exception paths are introduced for those sparse paths.
• Truth data added where simple EXPECT_* / assert checks are
  insufficient for algorithmic correctness.
  N/A: dense-vs-sparse equivalence and explicit expected sparse encodings are covered with direct checks.
• CI runtime impact considered; team notified if significant.

Documentation

• Public-facing APIs have Doxygen docs.
• User-visible behavior changes have public docs, or a follow-up is
  tracked.

Code style

• Naming follows the existing convention (snake_case vs camelCase) for
  the area being modified.

Dependencies

• No new third-party dependencies, or the team has been notified and
  OSRB tickets filed.