feat!: Removed model_unstable feature flag.

[zrho]

This PR removes the model_unstable feature flag across the repository. Closes #2104.

BREAKING CHANGE: Downstream crates need to remove the model_unstable feature flag when referencing hugr or hugr-core.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.58%. Comparing base (2222b8c) to head (c6d66e4).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2113      +/-   ##
==========================================
+ Coverage   83.08%   84.58%   +1.49%     
==========================================
  Files         219      221       +2     
  Lines       41886    45344    +3458     
  Branches    37961    41287    +3326     
==========================================
+ Hits        34803    38356    +3553     
+ Misses       5278     5254      -24     
+ Partials     1805     1734      -71     

Flag	Coverage Δ
python	86.17% <ø> (+0.46%)	⬆️
rust	84.43% <ø> (+1.61%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[hugrbot]

This PR contains breaking changes to the public Rust API.

cargo-semver-checks summary

--- failure feature_missing: package feature removed or renamed ---

Description:
A feature has been removed from this package's Cargo.toml. This will break downstream crates which enable that feature.
      ref: https://doc.rust-lang.org/cargo/reference/semver.html#cargo-feature-remove
     impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.41.0/src/lints/feature_missing.ron

Failed in:
feature model_unstable in the package's Cargo.toml
feature hugr-model in the package's Cargo.toml

--- failure feature_missing: package feature removed or renamed ---

Description:
A feature has been removed from this package's Cargo.toml. This will break downstream crates which enable that feature.
      ref: https://doc.rust-lang.org/cargo/reference/semver.html#cargo-feature-remove
     impl: https://github.com/obi1kenobi/cargo-semver-checks/tree/v0.41.0/src/lints/feature_missing.ron

Failed in:
feature model_unstable in the package's Cargo.toml
feature hugr-model in the package's Cargo.toml

[aborgna-q]

Please update the release group in release-plz.toml too.

hugr/release-plz.toml

Lines 66 to 67 in 2222b8c

	# Use a separate version group while the dependency is `-unstable`,
	# to avoid breaking releases of the main package.

[aborgna-q]

Note: The change has been reverted from main and put into a new PR targeting the release branch instead.