# 108 — Rework the engine test harness for the primary/profile actor split

## Summary

`src/engine/tests.rs` (2300+ lines) and the `test_handle` pattern are
built entirely around one actor with one channel and one snapshot.
After tickets 103-107 land, tests need to exercise three shapes that
don't exist today: the primary actor alone, the profile actor alone,
and primary+profile interaction (including lag/backfill scenarios).
This is explicitly called out as a gap in [[0001-actor-split]]'s "Not
yet considered" section — real rework, not just extension.

## Where

- `src/engine/tests.rs` — the whole file; most existing tests
  construct an `EngineSnapshot` directly or drive it through a single
  `test_handle`-obtained channel and assert against one snapshot's
  query results. Every test that declares a metric, ingests a
  matching line, and asserts on bucket/query output now spans two
  actors.
- `src/engine/handle.rs` `test_handle` (see ticket 099's history for
  the existing single-actor test-visibility pattern) — needs an
  equivalent for the profile actor, and likely a combined test harness
  helper that spins up both and wires them together the way `spawn`
  does in production.
- `src/api/mod.rs`'s test module (`ingest_lines`/similar helpers, e.g.
  around `:1817` per prior research) and `src/config/profile.rs`'s
  test module — both currently assume one engine handle; they need
  updating wherever they exercise metric/alert behavior that's now
  profile-actor-owned.

## Scope

- Add primary-only test helpers (construct a primary actor, ingest
  lines, assert on raw storage/param extraction only — no metrics).
- Add profile-only test helpers (construct a profile actor against a
  fake/minimal primary read-side, declare metrics, assert on bucket
  computation and alert evaluation in isolation).
- Add combined-harness helpers for tests that need real end-to-end
  behavior (ingest via primary, wait for the profile actor to catch up
  via the watch signal — needs a deterministic "wait until profile
  actor has processed up to sequence N" helper rather than a sleep, to
  avoid flaky tests) — a `barrier`-equivalent for the profile actor,
  mirroring `EngineHandle::barrier` (`src/engine/handle.rs:89-101` per
  prior research).
- Add explicit tests for the accepted lag/eviction tradeoff from
  [[0001-actor-split]]'s resolved decisions: primary evicts on
  schedule regardless of profile-actor progress, and a lagging profile
  actor silently misses data for the evicted range — this behavior
  should be asserted, not just assumed.
- Migrate existing tests in `src/engine/tests.rs` to the new harness
  incrementally; tests that only exercise primary-side behavior (param
  declaration, log storage, eviction) need minimal changes, while
  tests exercising metrics/alerts need the combined harness.
- Out of scope: adding new test *coverage* beyond what's needed to
  preserve today's behavior guarantees across the split — this is a
  harness migration, not a coverage expansion project.