feat: custom seroval plugins via serialization.plugins

[birkskyum]

• Closes [Feature?]: Custom Seroval plugins #1474

Values seroval has no built-in support for (Mongo ObjectId, Prisma Decimal, Temporal, custom classes) threw SerovalUnsupportedTypeError when crossing a server function boundary, with no way to register a plugin. Seroval has supported createPlugin since v1; SolidStart just hardcoded its plugin list.

// vite.config.ts
solidStart({
  serialization: { plugins: "src/seroval-plugins.ts" },
});

// src/seroval-plugins.ts
import { createPlugin } from "@solidjs/start/serialization";
export default [MyPlugin];

The path resolves through a new solid-start:seroval-plugins virtual module, wired like solid-start:middleware. One module feeds both fns/client.ts and fns/handler.ts, so client and server can't drift out of sync. Built-in plugins keep precedence (seroval takes the first passing test(), user plugins are appended).

Also adds a @solidjs/start/serialization entrypoint re-exporting createPlugin, OpaqueReference, and plugin types. Without it users need their own seroval dep, and a version mismatch wouldn't fail the build, it would emit plugin nodes the other end can't interpret.

Two limits, both documented on the option:

• Only server function and action payloads. The SSR hydration payload is serialized by solid-js/web and is unaffected.
• Under mode: "js" the payload is eval'd on the client, so a plugin's serialize may reference globals only, not the plugin module's imports. deserialize (mode: "json", the default) has no such limit.

Verified: 6 new unit tests, 1 new e2e round-tripping a custom class in both directions, and a manual browser check under both serialization modes. Full suites green (67 unit, 31 e2e, 31 bundled-dev).

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🦋 Changeset detected

Latest commit: a6eb444

The changes in this PR will be included in the next version bump.

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Name	Type
@solidjs/start	Minor

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[lxsmnsyc]

lgtm