Support Stim DEM strings in get_decoder

[vedika-saravanan]

Description

Adds unified decoder initialization from either a parity-check matrix or raw Stim detector error model text.

This supports both existing PCM-based decoders and DEM-native decoders needed by #546. PCM-based decoders can parse DEM text into H, O, and error_rate_vec defaults, while DEM-native decoders can consume the raw DEM text without going through a lossy matrix conversion.

API (cudaq/qec/decoder.h)

• Adds decoder_init = std::variant<sparse_binary_matrix, std::string> as the decoder registry construction input.
• Keeps existing PCM construction paths:
  • get_decoder(name, H, options)
  • decoder::get(name, H, options)
• Adds DEM string construction through the same entry points:
  • get_decoder(name, dem_text, options)
  • decoder::get(name, dem_text, options)
• Adds dem_from_stim_text(dem_text) to parse Stim DEM text into detector_error_model.
• Adds helper routing for PCM-based decoders so DEM-derived O and error_rate_vec are supplied as defaults when not explicitly provided by the user.

The DEM-to-detector_error_model parser is lossy: it extracts detector flips, observable flips, and per-error probabilities, but drops detector coordinates and separator-encoded correlation structure. DEM-native decoders should consume the raw string alternative in decoder_init.

Python: cudaq_qec.get_decoder(...) now accepts Stim DEM strings. Python-registered decoders currently receive parsed H plus DEM-derived O / error_rate_vec defaults, not raw DEM text.

Dependency / build

QEC now provides its own Stim dependency via FetchContent when libstim is not already available, so standalone QEC builds do not depend on parent CUDA-Q interim build artifacts.

Tests

C++ gtests and Python tests cover:

• DEM string construction through get_decoder(...)
• PCM construction still working
• user-provided options overriding DEM-derived defaults
• DEM parse edge cases
• DEMs without observables
• stim::DemTarget category assumptions

Out of scope / follow-ups

Chromobius plugin integration itself; detector-coordinate storage on detector_error_model; optional PyMatching-specific improvements; user-facing Sphinx/RST docs.

Runtime / performance impact

N/A

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Tests

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[bmhowe23]

This is looking good...thanks, @vedika-saravanan. Just a few follow-up questions below.

[bmhowe23]

Thanks! Could you please create a follow-up PR/issue to add docs for this capability? We should add a public example for this.

[vedika-saravanan]

GHI for adding docs and example #595

[bmhowe23]

@vedika-saravanan can you please do ninja zip_installed_files and inspect the zip file that it produces? I am seeing many stim binaries and directories that should not be "installed", and I suspect this PR is the one that added them.