test(backend): init/construct/runtime error-frame coverage

[gmaclennan]

Summary

PR #36 added the rootkey handshake ({type:"init",rootKey:"<base64>"}) and the structured error-frame broadcast on the control socket, but backend/index.js's validation and fatal-handling logic was untested. This adds that coverage with a surgical, behaviour-preserving refactor.

• Extracted the init-payload validation (string check, strict-base64-22 check, decode-to-16-bytes check) into a pure backend/lib/validate-init.mjs#validateInit that returns either the 16-byte Buffer or a structured error. The inline checks moved verbatim; index.js's init handler now calls the helper.
• Extracted the fatal error-frame construction into backend/lib/error-frame.mjs#errorFrame and made handleFatal's broadcast / captureFatal / flush / exit deps injectable (they default to the current module singletons, so runtime behaviour is unchanged).
• Added unit tests for both pure helpers and child_process.spawn-based integration tests that drive a fake control-socket client against the real loader.mjs -> index.js boot to assert the process-level error frames and non-zero exit.
• Wired the new top-level index.test.mjs into the node --test runner (backend/package.json test script).

The refactor is kept minimal because backend/index.js is also touched by the in-flight Sentry work — this should merge before the Sentry PR (both touch backend/index.js).

Acceptance criteria (#38)

• Runner wired — already node --test; confirmed and extended to include the new top-level index.test.mjs.
• Init malformed -> non-zero exit + phase "init" broadcast.
• Construct failure -> phase "construct" broadcast before exit.
• Uncaught throw -> phase "runtime" broadcast.
• Init handler unit coverage: non-string rootKey -> error; wrong-length-after-decode -> error; valid -> resolves; second init after consumption -> ignored.

Checks run

• npm --prefix backend ci --ignore-scripts then cd backend && node --test (and npm test): 43 pass / 0 fail (35 pre-existing + 8 new across validate-init.test.mjs and index.test.mjs).
• npm run types (backend tsc): my changes add no new errors. One pre-existing error remains — lib/install-polywasm.js(16): TS7016 'polywasm' has no declaration file — which is present on a pristine origin/main checkout (untouched file, missing @types) and unrelated to this PR.

Not run locally

• No native (Kotlin/Swift) suites are involved in this change, so nothing native to run.
• The integration tests spawn the real backend; they rely on @comapeo/core importing under Node 24, which it does in this environment. CI runs the same node --test.

Closes #38

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