feat!: make @mermaid-js/mermaid-cli an optional peer dependency

[keonik]

Closes #293
Closes #176

⚠️ Breaking — this is the 3.0 change. Ships as a major changeset.

What

mmdc is only ever invoked as an external binary (node_modules/.bin/mmdc), never imported. And for .md output generate returns long before anything touches it. So every user was installing the CLI — plus mermaid (~76 MB) and a headless Chromium via its puppeteer peer — whether or not they could ever use it.

It moves to an optional peer:

"peerDependencies": {
  "@prisma/client": "^5.0.0 || ^6.0.0 || ^7.0.0",
  "@mermaid-js/mermaid-cli": "^11.0.0"
},
"peerDependenciesMeta": {
  "@mermaid-js/mermaid-cli": { "optional": true }
}

Thanks @RPDeshaies and @mo-ayala for the original Docker-image write-up on #176 — this is the fix for that.

Migration

Rendering .svg / .png / .pdf — the default output is ./prisma/ERD.svg, so this is most people:

npm i -D @mermaid-js/mermaid-cli puppeteer

Rendering .md / .mmd: nothing to do, and you can now drop both packages.

The README install block, the output-extension table, and a "Upgrading to 3.x" section all say this.

Also in here

Small things that only make sense alongside the above:

• .mmd output — writes bare mermaid with no code fence. Remove puppeteer dependency when outputting mermaid markdown #176 asked for .md/.mmd; only .md existed.

• An actionable error. Previously a missing CLI gave Expected mermaid CLI at <path> or undefined but this package was not found. Now:

  prisma-erd-generator: could not find the mermaid CLI (mmdc).
  
  Rendering to .svg, .png or .pdf needs @mermaid-js/mermaid-cli, which is an
  optional peer dependency so that it is not installed for everyone:
  
      npm i -D @mermaid-js/mermaid-cli puppeteer
  
  Already installed? Point the generator at the binary instead:
  
      generator erd {
        provider = "prisma-erd-generator"
        mmdcPath = "node_modules/.bin"
      }
  
  Don't need an image? Markdown output renders with no browser at all:
  
      output = "../ERD.md"
  
  Searched <path> and `find ../.. -name mmdc`.
  

• CLI resolved before the puppeteer config is built. That config exists only to be passed to mmdc, so building it first meant a missing CLI got buried under the arm64 which chromium probe dumping a Command failed: which chromium stack trace first. Verified before/after in an isolated directory.

• find ../.. -name mmdc no longer crashes. It was an unguarded execSync, so on Windows (no find) or a missing parent it threw a spawn error instead of falling through to "not found" — plausibly part of Installation bug: reify:poppeteer-core: timing... #183. Now a failed search is just a miss.

Testing

• New __tests__/textOutput.test.ts — .md fencing, .mmd bare output, and a guard that @mermaid-js/mermaid-cli stays out of dependencies and stays marked optional, so this doesn't silently regress.
• Full suite (30 files / 35 tests) passes against Prisma 7. @mermaid-js/mermaid-cli moved to devDependencies, so CI and local runs still render real SVGs.
• Manually reproduced the missing-CLI path in an isolated directory with no node_modules to confirm the message and the ordering fix.