ci(coverage): establish test-coverage baseline for Node/Vite apps in src/500-application

Problem Statement

PR #633 introduced the first Node/TypeScript application (Vite/esbuild + Vitest) under src/500-application/: 516-chat-with-your-factory. The app already ships Vitest with the @vitest/coverage-v8 provider and passing unit tests, but nothing in CI runs those tests, enforces them, or reports coverage to Codecov.

Rust crates in the same folder have a complete coverage + registration + enforcement contract (see .github/instructions/rust-crate-registration.instructions.md). Node/Vite apps have no equivalent, so unit tests can silently rot and coverage is invisible. This gap will repeat for every future Node/Vite app unless a shared baseline is established.

Current State (gap analysis)

What 516 already has (local only)

• test, test:watch, test:coverage scripts and @vitest/coverage-v8 in package.json
• A vitest.config.ts with a v8 coverage provider (default text reporter only — no Cobertura/lcov output)
• At least one real test: src/server/factoryTool.test.ts

Proposed Solution

Build a generic baseline for all Vite/Node apps under src/500-application/, not a one-off for 516.

1. Reusable Node test/coverage workflow (e.g. .github/workflows/node-tests.yml)

   • Matrix over registered Node packages under src/500-application/
   • npm ci → npm run test:coverage → upload Cobertura/lcov to Codecov under a new js (or node) flag
   • Standardize a coverage reporter (Cobertura/lcov) in each app's vitest.config.ts (516 currently emits only the default text reporter) and a consistent test:coverage script convention

2. PR gating in pr-validation.yml

   • Add a changesInNode-style output to scripts/build/Detect-Folder-Changes.ps1 so the workflow runs only when Node app files (or the workflow/codecov config) change

3. Codecov registration in codecov.yml

   • Add flags.js with per-app paths, plus coverage.status.project/patch entries, or an explicit ignore opt-out path — matching the Rust flag model

4. Registration enforcement gate

   • New validator workflow + script (mirroring Validate-RustCrateRegistration.ps1) that fails a PR when a Vite/Node app under src/500-application/ is neither fully registered (workflow matrix + codecov flag) nor explicitly opted out

5. Authoring instructions

   • New node-package-registration.instructions.md with applyTo: **/src/500-application/**/package.json (+ workflow/codecov globs), documenting the registration/opt-out contract, analogous to the Rust instructions

Benefits

• Node/Vite apps get the same coverage visibility and enforcement guarantees as Rust crates
• Prevents silent test rot and untracked coverage as more Node apps land under src/500-application/
• Establishes a documented, self-service registration path for future app authors

Acceptance Criteria

• Reusable Node test/coverage workflow runs vitest with coverage for registered apps and uploads to Codecov under a dedicated flag
• 516-chat-with-your-factory is registered and reporting coverage as the first consumer
• Node tests are PR-gated on Node-relevant changes via Detect-Folder-Changes.ps1
• codecov.yml has a Node/JS flag with paths + project/patch status (and an opt-out path)
• A CI gate fails PRs that add an unregistered/opted-out Vite/Node app
• Instruction file documents the registration and opt-out process
• A standard test:coverage script + coverage reporter convention is documented for new apps

Notes / Open Questions

• Flag naming: reuse js to align with the existing fuzz-js-* scheme, or use node?
• Node version: standardize on 24 (used across existing workflows) vs. per-app engines
• Should coverage thresholds start informational: true (like Rust patch) to avoid blocking early adopters?

Related Issues

• Prerequisite implementation slice for the JS/TS stack of #175 (ci(coverage): enforce 80% coverage thresholds across all language stacks). #175 assumes vitest coverage is already produced and uploaded for JS/TS code, which does not yet exist for src/500-application/ Node apps. Recommend tracking this as a dependency of #175.
• #140 (add coverage thresholds to frontend vitest configuration, closed) covered docs/ docusaurus JS only — no overlap.
• #155 (Rust coverage) and #455 (extract -core crates from WASM operators) are the Rust-side precedent this issue mirrors — no functional overlap.
• #144 (docs(testing): update testing-validation.md) is documentation-only and would consume the conventions this issue establishes.

Start with .github/instructions/rust-crate-registration.instructions.md, rust-tests.yml, application-matrix-builds.yml, pr-validation.yml, codecov.yml, and scripts/build/Detect-Folder-Changes.ps1. Inspect 516-chat-with-your-factory/package.json and vitest.config.ts, then compare its setup with the Rust registration and coverage flow. Done means registered Node coverage runs in CI, reports to Codecov, is change-gated, and has validation and authoring instructions.