Several per-request hot paths iterated secret providers with sequential
yield*, turning a fan-out into N round-trips. Fix by switching to
Effect.all(..., { concurrency: 'unbounded' }).

- executor.ts secretsGet fallback: when the core routing-table misses,
  ask every enumerating provider in parallel; first non-null in
  registration order wins. Providers that throw are still treated as
  'don't have it'.

- executor.ts secretsRemove: delete across every writable provider in
  parallel. Providers don't coordinate on ownership — each gets asked,
  and most calls are no-ops.

- executor.ts secretsList: provider.list() runs in parallel, then the
  results are merged in registration order so 'first provider wins'
  precedence remains deterministic.

- openapi/invoke.ts + graphql/invoke.ts resolveHeaders: resolve
  secret-backed headers in parallel. OpenAPI fails the invocation on
  any missing secret (unchanged semantics); GraphQL drops the header
  silently on failure (unchanged semantics).

N is small (3-5 providers, 1-5 headers) so parallelism is unbounded;
no Effect.all concurrency cap needed.