feat(parity): reuse verified build artifacts

[TheHypnoo]

Summary

• add explicit fixture-only reuse of a verified Perry compiler and matching runtime archives
• keep the default release build path unchanged
• reject invalid reuse configuration and document artifact freshness responsibility

Changes

• run_parity_tests.sh: add PERRY_SKIP_BUILD=1, validate PERRY_BIN plus one native runtime/stdlib pair, and force the existing no-auto archive path
• tests/test_parity_build_reuse.sh: cover invalid configuration, incomplete artifacts, caller binary preservation, runtime binding, and runner-owned Cargo suppression
• CONTRIBUTING.md: document fixture-only reuse and rebuild boundaries

Related issue

Closes #7062

Test plan

• bash -n run_parity_tests.sh tests/test_parity_build_reuse.sh
• shellcheck -S warning run_parity_tests.sh tests/test_parity_build_reuse.sh
• tests/test_parity_build_reuse.sh
• git diff --check
• verified the default path still invokes the original Cargo build command
• ./scripts/pre-tag-check.sh --quick — existing main failures remain in workspace architecture, public benchmark freshness, file-size, GC store-site, and address-classification audits; none touch this PR's files

Checklist

• I have NOT bumped the workspace version or edited CLAUDE.md / CHANGELOG.md
• My commit follows the repository's conventional prefix style
• I've read CONTRIBUTING.md

Summary by CodeRabbit

• New Features

  • Added support for running parity tests with prebuilt compiler and runtime artifacts, skipping compilation when configured.
  • Added validation and clear errors for missing or invalid prebuilt artifacts.

• Documentation

  • Documented how to run fixture-only parity tests and when a rebuild is required.

• Tests

  • Added coverage for prebuilt artifact reuse, validation failures, platform-specific archives, and ensuring builds are skipped.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro Plus

Run ID: 34654bee-3936-4e8f-b8be-39b6fc979076

📥 Commits

Reviewing files that changed from the base of the PR and between 788a757 and 666be4d.

📒 Files selected for processing (3)

• CONTRIBUTING.md
• run_parity_tests.sh
• tests/test_parity_build_reuse.sh

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📝 Walkthrough

Walkthrough

The parity runner now supports PERRY_SKIP_BUILD=1 to validate and reuse a supplied compiler plus matching runtime and stdlib archives. Cargo builds are skipped in this mode, with regression coverage and contributor documentation added.

Changes

Parity artifact reuse

Layer / File(s)	Summary
Prebuilt artifact validation run_parity_tests.sh	Validates PERRY_SKIP_BUILD, PERRY_BIN, and platform-specific runtime/stdlib archives, then configures the runtime directory and disables auto-optimization.
Build gating and status reporting run_parity_tests.sh	Skips compiler and optional WebAssembly host builds in reuse mode and reports whether artifacts were built or verified.
Reuse regression coverage and documentation tests/test_parity_build_reuse.sh, CONTRIBUTING.md	Tests invalid configurations and successful reuse without Cargo invocation, and documents the reuse command and rebuild conditions.

Estimated code review effort: 3 (Moderate) | ~20 minutes

Sequence Diagram(s)

sequenceDiagram
  participant Contributor
  participant run_parity_tests.sh
  participant PrebuiltArtifacts
  participant FixtureSuite
  Contributor->>run_parity_tests.sh: set PERRY_SKIP_BUILD=1
  run_parity_tests.sh->>PrebuiltArtifacts: validate PERRY_BIN and runtime/stdlib archives
  run_parity_tests.sh->>FixtureSuite: run parity fixtures without Cargo builds
  FixtureSuite-->>Contributor: report parity results

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Possibly related PRs

• PerryTS/perry#6891: Both changes modify parity runner archive and build gating logic involving PERRY_NO_AUTO_OPTIMIZE.

Suggested labels: parity

Suggested reviewers: proggeramlug, andrewtdiz

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly describes the main change: enabling reuse of verified build artifacts for parity runs.
Description check	✅ Passed	The PR description covers summary, changes, related issue, test plan, and checklist, so it is mostly complete.
Linked Issues check	✅ Passed	The changes implement the requested opt-in reuse mode, validation, default-path preservation, and docs/tests for #7062.
Out of Scope Changes check	✅ Passed	All listed file changes support the artifact-reuse feature; no unrelated code changes are apparent.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch feat/parity-build-reuse

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