feat: register Iconify icon packs so diagram icons render

[dariodd]

The problem

mermaid ships no icons beyond architecture-beta's five built-ins (cloud, database, disk, internet, server). Every other icon — logos:aws, mdi:database, an icon: on a flowchart node — is resolved against packs the host registers, and the Live Editor registers none.

So today, in the Live Editor:

architecture-beta
  service a(logos:aws)[AWS]
  service b(server)[Server]

b renders. a renders as mermaid's blue "?" placeholder — as does every icon in the icon documentation's own examples, and every icon in a diagram brought here from a tool that does register packs.

The change

Register eight common Iconify packs with CDN loaders, exactly as the documentation describes:

mermaid.registerIconPacks(
  ICON_PACKS.map((name) => ({
    name,
    loader: () =>
      fetch(`https://unpkg.com/@iconify-json/${name}@1/icons.json`).then((res) => res.json())
  }))
);

logos, simple-icons, mdi, fa6-solid, fa6-brands, carbon, tabler, devicon.

Cost

Nothing until an icon is used. mermaid calls a loader only when a diagram names that pack, and caches the result — so a session that draws no icons makes no request, and one that uses logos: fetches logos alone. Nothing is added to the bundle and there is no new dependency.

If a fetch fails — offline, or a self-hosted instance with no outbound access — mermaid falls back to the same "?" placeholder shown today. Nothing regresses.

Alternatives considered

• Bundling the packs (@iconify-json/* as dependencies with import() loaders) keeps it offline, but mdi alone is 3.1 MB and the eight together are far more; a CDN fetch on demand seemed the better trade for a hosted editor. Happy to switch if you would rather not depend on unpkg.
• A different pack list. These eight are a guess at what people reach for; glad to trim or extend it.

Tests

Not included on purpose: the only meaningful assertion is "an icon rendered", which needs a real network fetch and would make CI flaky. Happy to add a Playwright test behind a mock if you would like one.

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