[yota88]

Why

The server now accepts a new variant on an existing union, but only when the schema says which version added it (variantMetadata on UnionDef, in api-gateway 1.1885.0). type-gen never sent it, so adding a variant to a deployed union always fails.

Changes

• IR: resolveSchemaChain records the version each union variant first appears in. convertUnionDefToIr writes variantMetadata for the variants added after the union.
• Sent schema: convertIrToWireSchema passes variantMetadata on, so ir asset, ir update-schema, and ir deploy send it. ir deploy --first-party already copies the whole union.
• Existing schemas: a union that never gained a variant gets no variantMetadata, so its IR and asset don't change.
• Minor versions: pack.schema now lets a minor version add union variants. Older apps on the same major read a new variant as unknown and keep it when they save. Removing a variant is still rejected.
• Write version: the union registry in generated SDKs now lists variantAddedInVersions, and state.core tags a write that uses an added variant with at least that version. Before, writing a variant added in v2 whose record existed in v1 was tagged v1.
• sdkgen templates: the "./*" export now points at build/esm/public/*.js and build/cjs/public/*.cjs, like the other packages. This was left out of Generate the Pack platform SDK in PACK instead of installing it from npm #342.

Don't merge until api-gateway and backpack copy variantMetadata through to schema validation. Until backpack reads it from first-party assets, an asset with an added variant fails to load. The generated platform SDK already has the field, so it doesn't need to be regenerated.

Flow

schema.mjs    nextSchema(v1, { bump: "minor" }): Shape.addVariant("square", Square) → 1.1
   │ type-gen schema ir
   ▼
IR (1.1)      Shape.variantMetadata: { square: { addedInVersion: 1, addedInMinorVersion: 1 } }
   ├─ ir asset / ir update-schema ─▶ api-gateway UnionDef.variantMetadata ─▶ backpack
   └─ ir deploy --first-party ─────▶ backpack UnionDef.variantMetadata