[Feature] Export Mailbox, Envelope and MailboxFactory — the documented mailbox escape hatch is unreachable

[pathosDev]

Problem

src/util/Constants.ts:78-83 tells users "Opt out per-actor via Props.withMailbox(() => new Mailbox())" — but Mailbox lives in src/internal/Mailbox.ts and is not re-exported: src/mailbox/index.ts exports only BoundedMailbox/PriorityMailbox, and the root barrel mirrors that (src/index.ts:697-712). A custom mailbox author also needs Envelope (unexported) and MailboxFactory (declared at src/Props.ts:7; the barrel exports ActorFactory and PropsConfig but not it, src/index.ts:66-67). The documented escape hatch is unreachable.

Same file: Props.withMailboxCapacity is silently ignored whenever withMailbox is set — src/internal/ActorCell.ts:154-165 only reads mailboxCapacity in the else branch.

Proposed behaviour

Re-export Mailbox, Envelope, MailboxFactory. Make withMailbox + withMailboxCapacity either compose or throw at Props construction.

Acceptance criteria

• The snippet in Constants.ts compiles against the published barrel.
• A test asserts the conflicting-Props error (or the composed behaviour).

Related: #414, #556

[pathosDev]

Re-derived independently in the production-readiness review tracked in #913. One detail that raises the priority here: the framework's own source promises this escape hatch and it does not exist.

src/internal/ActorCell.ts:190-194
      // #310 — bounded by default.  Unbounded was the pre-#310 default
      // and is still available via `withMailbox(() => new Mailbox())`
      // for use-cases that need it (deterministic replay, test setups,
      // tightly-controlled throughput).

Mailbox is exported from neither src/index.ts nor src/mailbox/index.ts, and package.json#exports declares only ., ./testkit, ./devtools and ./package.json — so there is no subpath to reach it through either. A package consumer therefore cannot opt out of a default that silently discards the oldest messages; they can only change the capacity or the overflow policy.

Measured on the default configuration: 100 000 tells to one actor, 9 999 processed, 90 001 discarded with no dead letter (see #773) and no log. That makes this a correctness escape hatch rather than an ergonomics export.