feat: Frontend support for Helios platform configuration

[cgh-qtnm]

This PR adds a new _set_platform_config function to tket-py under the path tket.platform.helios. This function sets some metadata on the root of a compiled HUGR which controls Helios-specific hardware features. The metadata object and key are defined in tket_qsystem::extension::qsystem::helios. Code to consume and apply the platform configuration is not included here.

As noted in the function docstring, this interface is unstable and subject to change without notice.

This is ported from the original implementation in Quantinuum/guppylang#1807.

[codecov]

Codecov Report

❌ Patch coverage is 90.90909% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.26%. Comparing base (c7f6ed4) to head (e00a4f1).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
tket-qsystem/src/extension/qsystem/helios.rs	0.00%	3 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1848   +/-   ##
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  Coverage   86.26%   86.26%           
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  Files         200      202    +2     
  Lines       31964    31996   +32     
  Branches    30514    30517    +3     
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+ Hits        27573    27602   +29     
- Misses       3159     3162    +3     
  Partials     1232     1232           

Flag	Coverage Δ
python	92.11% <100.00%> (+0.16%)	⬆️
rust	85.98% <0.00%> (-0.01%)	⬇️

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[Copilot]

Pull request overview

Adds a Python-facing API for attaching Helios-specific platform configuration metadata to compiled HUGRs, with the metadata key/value type defined in the tket-qsystem Helios extension and surfaced through tket-py.

Changes:

• Introduce HeliosPublicConfig as a serializable metadata payload type with a dedicated metadata key in tket-qsystem.
• Add tket.platform.helios.set_platform_config to write the configuration metadata onto HUGR module roots (for both Hugr and Package).
• Export the metadata key through the Python native bindings and add Python tests covering defaults, custom values, and per-module independence.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
tket-qsystem/src/extension/qsystem/helios.rs	Adds HeliosPublicConfig metadata type/key and a serde roundtrip test.
tket-py/tket/platform/helios.py	Implements set_platform_config and exposes HELIOS_CONFIG_META_KEY.
tket-py/tket/platform/init.py	Introduces the tket.platform package and re-exports helios.
tket-py/tket/_tket/metadata.pyi	Adds stub for the exported HELIOS_CONFIG_KEY constant.
tket-py/tket/init.py	Exposes platform at the top-level tket package.
tket-py/test/test_platform_helios.py	Adds unit tests for Helios platform configuration metadata behavior.
tket-py/src/metadata.rs	Exports HELIOS_CONFIG_KEY from Rust into the Python bindings.

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[cgh-qtnm]

With the addition of leakage_repump the set of supported config features is now complete. I think this PR can move independently of the backend support, it won't break anything if we release this first.

[aborgna-q]

This should be a Metadata subclass instead, plus a python enum for the options. Add the definition to metadata.py.

[cgh-qtnm]

I made it a metadata subclass, but I don't understand what you mean about the enum for the options.

[aborgna-q]

Sorry, "dataclass" rather than enum.

We can use a specialized value type for the python Metadata protocol instead of a generic dict[str, bool].