0.5.1

Fixed

• The browser lab runs again. CreateInvoice took an account_id from 0.5.0 and the lab's
  script did not send one, so the module refused the first command of the run —
  {"kind":"undecodable","at":"input.account_id","expected":"a value","found":"nothing"} — and
  task site-build failed after the tag was cut. The script now names the account its invoices
  belong to, which is the caller's to name because billing.invoice.Account declares it owns
  them via account_id.
• The lab read the identity for the next command out of the first entity in the catalogue, which
  was the invoice only for as long as the invoice was the only entity. billing.invoice.Account is
  declared above it, so InvoiceCreated's account_id was taken for the new invoice's id and the
  next command was issued against an account. It now reads the identity of the entity the taken
  outcome creates:.
• website/src/pages/lab/_source.ts, the lab's committed copy of examples/billing/domains/invoice.yaml,
  was not refreshed for 0.5.0, so every line the left panel highlighted was off by the relation's
  own lines. Refreshed — and a stale copy is now a red gate rather than a wrong panel:
  task site-lab compares the two.
• examples/billing-web/smoke.mjs asserted the pre-relation input list and sent the pre-relation
  input; it is run by no task, which is why nothing caught it. Same fix, and it passes.
• The documentation pages that quote the command were carrying its old shape:
  specification-to-contracts.md said three values are chosen where there are now four and quoted
  a JSON Schema "in full" that was missing account_id, and write-a-specification.md showed the
  payload block without its first line. Both are the generated files again.