Break ticket 102's actor split into reviewable sub-tickets (103-108)

Ticket 102 (splitting the engine into reader/primary/profile actors)
was a single multi-thousand-line ticket covering a new actor
topology, an EngineHandle API redesign, a query fan-out layer, moving
alerts, and a full test-harness rewrite, with some design decisions
deliberately left open. Splitting it into six sequenced tickets
(profile actor skeleton + signaling, move metric computation, move
alert evaluation, split the handle API, query fan-out, test harness
rework) lets each land as its own reviewable commit instead of one
large speculative rewrite.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TeyhRfMYjMditek6geDRoQ

tickets/108-rework-engine-test-harness-for-actor-split.md

@@ -0,0 +1,57 @@
+# 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.