Name the operation on mailbox change events

[TheGreatAxios]

Summary

The live change event carried only an id — a listener could not tell a
read from an archive from an assignment without re-fetching and
diffing against remembered state.

• Add an optional MailboxEventOp (create / mark_read /
  mark_unread / trash / archive / restore / enrich /
  assign) to MailboxEvent.
• Thread it through every publishMailboxEvent call site: the two
  delivery paths (writeMailboxMessage, deliverInboxItems) and the
  transport dual-write (createMailboxPersist) all publish create;
  mountMailbox's mutation routes publish their own action, reusing
  the same identifiers applyMailboxBulkAction already defines so a
  single-message mutation and a bulk one report the same op.
• Additive, not a reshape: op is optional on
  MailboxEventSchema and on publishMailboxEvent. An existing
  listener that only reads id, or an existing caller of
  publishMailboxEvent that omits op, is unaffected.
• Document delivery semantics that were previously unstated — events
  can be missed, duplicated, or arrive out of order — in
  MailboxEventBus's doc comment and the README's SSE client
  contract.

Scope note

No overlap with #8 (frame-size / recipient-fan-out caps) — that PR
does not touch bus.ts, the publish call sites, or mount.ts's route
table. Both PRs touch the Unreleased section of CHANGELOG.md and add
prose to ARCHITECTURE.md/README.md; expect a straightforward textual
merge, not a semantic one.

Verification

Local bun install / bun test was not run (sandboxed dev machine,
OOMs on install/build) — new and touched tests are written to the
repo's existing patterns and are expected to pass under CI:

• packages/mailbox/src/bus.test.ts (new) — schema accepts the
  pre-existing { type, id } shape and a known op, rejects an
  unknown op; publishMailboxEvent omits op when not given one
  and includes it when given one.
• packages/mailbox/src/mount-event-op.test.ts (new) — each single
  mutation verb, enrich, assign, and bulk publish their expected
  op over a real mounted app.
• write.test.ts / persist.test.ts — extended existing bus-publish
  tests to assert op === "create".
• examples/reference-host/test/acceptance.test.ts — updated the one
  exact-JSON SSE assertion to include the new op field.

Closes CL-5018

[TheGreatAxios]

Rebased onto main now that #8 is merged. Clean rebase, no conflicts — #8 never touched bus.ts or mount.ts, and its frame-size/recipient guardrails in write.ts/persist.ts sit entirely before the existing publishMailboxEvent call sites this PR modifies, so there's no functional overlap. Re-checked git diff against #8's merge commit to confirm the three publish call sites (write.ts x2, persist.ts) are unchanged by it.

Two notes for review:

• Additive-ness is visible directly in the bus.ts diff, not just the description: publishMailboxEvent's body is op ? { type: "mailbox", id, op } : { type: "mailbox", id } — omitting op reproduces the exact original two-field shape, byte for byte. bus.test.ts asserts this ("omitting op publishes the original two-field shape").
• This PR does not touch the transport-write-swallows-refusal behavior a reviewer flagged separately (tracked elsewhere, blocking the HTTP delivery route work, not this PR). Worth noting explicitly: if that write is refused, announce/publish is never reached — no event fires at all, so a listener sees nothing rather than a wrong op. Unchanged by this PR either way.