Tests: split tests/ into unit/ + integration/ subtrees

[pathosDev]

Spawned from the coverage analysis after the chat-sample sweep.
Hygiene refactor — no behavioural change, no new tests, no
removed tests.

The gap

tests/integration/ is referenced conceptually but empty:

tests/unit/         164 test files
tests/integration/    0 files
tests/multi-node/  ~14 test files (cluster, multi-process)
tests/smoke/         1 file (cross-runtime smoke)
tests/util/          3 helper files

The current convention is "everything that runs in-process under
Bun goes in tests/unit/", but several files already break that
contract — Cassandra/Sqlite tests touch real databases, the
broker tests spin up real Kafka/MQTT/Redis containers when
*_LIVE=1, the multi-node tests fork worker threads.

When we land #293 (multi-runtime CI), #296 (testcontainer broker
suites), and #297 (S3 LocalStack), we'll be adding many more
tests that legitimately don't belong in tests/unit/. Without
the split, tests/unit/ becomes a meaningless catch-all.

Scope

• Move from:

  tests/unit/{cluster,persistence,io,...}/...
  tests/multi-node/...
  

  to:

  tests/unit/         — pure in-process tests, no external services
  tests/integration/  — multi-process, containers, real cloud SDK paths
  

• File moves only — no test-code edits. Suggested mapping:
  • tests/multi-node/** → tests/integration/cluster/
  • tests/unit/cluster/sharding/CassandraRememberEntitiesStore.test.ts → tests/integration/persistence/
  • tests/unit/persistence/CassandraJournal.test.ts → tests/integration/persistence/
  • tests/unit/persistence/CassandraTagIndex.test.ts → tests/integration/persistence/
  • tests/unit/io/broker/*Actor.test.ts (the ones with *_LIVE=1 opt-in containers) → tests/integration/io/
  • tests/unit/coordination/KubernetesLease.test.ts → tests/integration/coordination/
• Add separate package.json scripts:
  • bun test tests/unit — fast path, runs in PR CI
  • bun test tests/integration — slow path, runs nightly or
    on-demand
  • bun test — both (current behaviour)
• Update CI to run them in separate jobs so failures are
  pre-attributed.

Acceptance criteria

• tests/integration/ contains all tests that touch real
  external services, multi-process workers, or cross-runtime
  I/O.
• tests/unit/ runs in < 30 s on a developer laptop.
• bun test still runs both groups by default.
• No test was removed or skipped during the move — only
  relocated.

Out of scope

• Renaming individual tests for clarity. This is purely the
  directory split.
• Restructuring tests/util/ — those are helpers, not tests.

Notes

This issue should land before #296 (broker testcontainers),
#297 (S3 LocalStack), and #298 (K8s providers) — those would
otherwise add 10+ new files into a directory whose semantics are
about to change.