Add frozen provider eval bundles and regression comparisons for S4

[chrishonson]

Summary

Adds the first slice of the S4 provider evaluation: local tooling that compares current MetaCortex retrieval with a self-hosted open-source provider (Onyx) using frozen eval bundles and regression comparison files. The doc opens with the question this answers and what exists so far.

• functions/eval/s4/ holds a frozen bundle (nine synthetic sources, eight behavior specs, five retrieval queries) and two scripted example result files. No private corpus and no credentials.
• functions/src/providerEvaluation.ts and providerResultAdapters.ts define the provider boundary, the runner, result normalizers for MetaCortex and Onyx, and the comparison report. They reuse computeRetrievalEvalMetrics, so there is one ranking metric implementation.
• functions/scripts/provider-eval.ts is a new entry point for the file commands, run with npm run eval:files. It never loads env files or initializes Firebase. The existing retrieval-eval.ts and its Firestore commands are unchanged from release.
• docs/s4/provider-evaluation.md describes the design and its limits. All eight labels are still needs_review and no behavior spec has run against a real worker.

No runtime code changes: nothing under the server entry points imports the new modules, so no deploy is needed.

Other changes

• functions/tsconfig.scripts.json also type-checks the three new S4 test files, so there is no separate S4 typecheck and verification.json is unchanged from release.
• scripts/check-journey-kit-install.mjs skips eval:files when it asserts that every script in package.json is packaged. The S4 tooling is offline and deliberately not shipped in the Journey kit.
• functions/src/retrievalEvaluation.ts labels synthetic cases synthetic_definition instead of implicit_fetch.

Test plan

Run from this branch with python3 scripts/run-gate.py <gate>:

• metacortex_typecheck passes
• metacortex_test passes: 14 files, 164 tests, 79.37% line coverage
• metacortex_package passes (Journey kit install check, 70 files)
• npm --prefix functions run eval:files -- validate-bundle --bundle eval/s4/bundle.json validates the frozen bundle
• CI on this PR

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