feat(ci): rehearse the monad fixture releases before they are cut

[pdobacz]

Collect and fill each feature on PRs, from-upstream and eips/**.

Aiming to reduce churn with botched tests releases when working on upstream changes and per-feature (per-EIP) fixture releases. This is part of an effort to enable CL to implement EIPs in independent branches, with spec tests coverage.

Tested here and here:
https://github.com/monad-developers/execution-specs/actions/runs/32856601361
https://github.com/monad-developers/execution-specs/actions/runs/32844861205

Greptile Summary

The PR adds an automatic fixture-release rehearsal workflow that derives its collection and fill matrices from the existing release configuration while discarding rehearsal artifacts.

• Adds branch-aware feature selection and matrix generation shared with release tooling.
• Runs collection checks before full fixture fills on pull requests and release-related branches.
• Adds an upload toggle to the fixture-building action and removes the unused mainnet release configuration.

Confidence Score: 5/5

The PR appears safe to merge; no concrete blocking or independently actionable non-blocking issue remains.

The generated matrices match the fixture action’s inputs, current configured features have applicable tests, unmatched EIP feature names safely fall back to all features, and artifact suppression covers both upload paths.

Important Files Changed

Filename	Overview
.github/scripts/check_release_matrix.py	Adds branch-aware feature selection and flattens the existing release matrix format for workflow consumption; no actionable defect was established.
.github/workflows/check_release.yaml	Adds setup, collection, and full-fill rehearsal jobs using the shared release configuration and fixture-building action.
.github/actions/build-fixtures/action.yaml	Adds an upload switch that suppresses both artifact-upload paths during rehearsals while preserving fixture generation.
.github/configs/feature.yaml	Removes an orphaned mainnet feature from a fork whose release workflow is explicitly limited to Monad features.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
    Trigger[PR, release-branch push, or manual dispatch] --> Setup[Select configured features]
    Setup --> Matrix[Build and flatten release matrices]
    Matrix --> Collect[Collect tests for each feature]
    Collect --> Decision{Full rehearsal?}
    Decision -->|No| Done[Report collection result]
    Decision -->|Yes| Fill[Run build-fixtures for each matrix entry]
    Fill --> Discard[Skip artifact upload]
    Discard --> Done

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_{Reviews (1): Last reviewed commit: "feat(ci): rehearse the monad fixture rel..." | Re-trigger Greptile}

[QEDK]

LGTM.