Collect/dispatch transaction buffer for coordinating async work

[Mateusz7410]

Motivation

When several services in one transaction each want to enqueue async work, they
either enqueue separately (scattered enqueues, no coordination) or have to thread
a shared builder through every call. A transaction-scoped buffer that collects
jobs and enqueues them once at the orchestration boundary would keep trigger-side
services declarative: each service states the work it needs, the orchestrator
decides when it is created.

Proposal

// from service A
Async.collect(new SendWelcomeJob(contacts));
// from service B
Async.collect(new ReviewContactJob(contactIds));
// once, at the trigger handler / service boundary
Async.dispatch();

• collect buffers QueueableJob instances in the current transaction.
• dispatch enqueues everything collected as one chain and clears the buffer.
• Optional dedupe by a caller-supplied key, so the same logical job collected
  twice collapses to one.

Notes

• Complements the existing incremental Async.queueable() builder; this is a
  static, transaction-scoped buffer any service can add to without holding a
  builder reference.
• dispatch on an empty buffer is a safe no-op.

Acceptance criteria

• New static entry points; existing global signatures untouched.
• Apex tests: multi-service collect, single dispatch, optional dedupe, empty
  dispatch no-op.
• Docs.