Updates a builder's display name, and its slug while that slug is still the auto-assigned builder-<id> placeholder. Authorised by a signature from the builder's on-chain getBuilderInfo(builder_id).fee_recipient — no gas is spent.

name is always editable. slug is set-once: once a real slug is stored, a request carrying a different slug is rejected; repeating the current slug is a no-op.

Typed data:

UpdateBuilderMetadata(address account,uint16 builderId,string slug,string name,uint256 nonce,uint32 deadline)

Nonce from GET /v1/auth/nonce, domain from GET /v1/auth/eip712-domain, same as RegisterBuilder.