fix(corpus): make standalone Corpus test self-contained

[Nelsonifenna]

Summary

This PR makes the Corpus test suite independently runnable from a clean checkout.

Problem

packages/corpus/package.json previously built only the schema package before running Corpus tests. However, the Corpus integration tests depend transitively on the built receipt and metadata packages.

As a result, the standalone command:

pnpm --filter @ever-guild/proof-runner-corpus test

could fail on a clean checkout, while the root test suite could hide the problem because other workspace pretest/build steps had already produced the required artifacts.

Solution

• Updated the Corpus pretest script to build schema, receipt, and metadata before running the tests.
• Added a CI regression guard that runs the standalone Corpus test immediately after pnpm install --frozen-lockfile, before other workspace builds can mask the dependency.

Testing

Verified that the standalone Corpus test passes from a clean dist/ state:

• 5/5 test files
• 37/37 tests

Also verified the relevant lint, typecheck, build, and root test commands without introducing regressions.

Issue

Closes #43

[ilyar]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Delightful!

Reviewed commit: 86ab06375e

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[ever-guild-ops]

No blocking issues found.

Scope reviewed: .github/workflows/ci.yml and packages/corpus/package.json against open head 86ab06375e... (merged against main).

Observed behavior and checks:

• Confirmed reported issue is fixed: on base 13b061d... pnpm --filter @ever-guild/proof-runner-corpus test fails with unresolved @ever-guild/proof-runner-metadata.
• On PR head, pnpm --filter @ever-guild/proof-runner-corpus test passes (5 files, 37 tests).
• pnpm test is currently blocked by environment (missing Playwright Chromium headless shell at runtime); this is an external test infra limitation and not caused by this change.
• In the same snapshot, pnpm typecheck, pnpm lint, and pnpm build all passed.

Given no PR-introduced correctness, security, or acceptance blockers remain, this is approvable.