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/106-split-enginehandle-api-for-primary-and-profile.md

@@ -0,0 +1,62 @@
+# 106 — Give API handlers a real way to address the primary vs. the profile actor
+
+## Summary
+
+By the time [[104-move-metric-computation-to-profile-actor]] and
+[[105-move-alert-evaluation-to-profile-actor]] land, metric/alert
+messages need to route to the profile actor while param/log messages
+stay on the primary — today's single `EngineHandle` (one `mpsc::Sender`
++ one `IndexReader`) can't express that. This ticket gives `api.rs`'s
+handlers, and the runtime profile-switch path, a real API surface for
+both actors, per [[0001-actor-split]]'s resolved decision to decide
+during implementation whether that's two handle types or one
+internally-fanning-out handle.
+
+## Where
+
+- `src/engine/handle.rs:74-78` (`EngineHandle { tx, reader }`) — the
+  single handle type `ApiState` holds today.
+- `src/api/mod.rs` — `ApiState`'s definition (wherever it stores the
+  `EngineHandle`) and every handler that sends an `EngineMessage`;
+  metric/alert-related routes need to target the profile actor's
+  channel instead of the primary's.
+- `src/api/mod.rs:812-852` (`reload_engine_declarations`,
+  `activate_profile`) — today removes/redeclares params *and* metrics
+  against one engine. After this split, param removal/redeclare stays
+  primary-side, metric removal/redeclare targets the profile actor —
+  this function needs to send to both.
+- `src/config/profile.rs`'s `load_into_engine` (referenced in
+  [[0004-profile-switch-reindexing-cost]] at `:332-384`) — same
+  dual-target concern at startup load time as well as runtime
+  reactivation.
+- `src/api/mcp_server.rs` — check whether the MCP server's query
+  surface holds its own `EngineHandle` reference that also needs the
+  updated shape.
+
+## Scope
+
+- Decide and implement the concrete shape: either (a) `ApiState` holds
+  two handles (`primary: EngineHandle`, `profile: ProfileHandle`) and
+  each handler picks the right one, or (b) one combined handle type
+  that internally holds both channels/readers and routes by message
+  kind. Document the choice in the PR description since
+  [[0001-actor-split]] deliberately left it open.
+- Update every `api.rs` handler that currently sends a
+  `DeclareMetric`/`UpdateMetric`/`RemoveMetricDeclaration` or alert-
+  related message to target the profile actor; param-related messages
+  (`DeclareParam`/`RemoveParamDeclaration`/`ClearParams`/
+  `SetMaxLogs`/`EnsureSortHelperIndex`) stay on the primary.
+- Update `reload_engine_declarations`/`activate_profile` and
+  `Profile::load_into_engine` to split their remove-then-redeclare
+  sequence across both actors correctly, preserving today's behavior
+  (still fully synchronous from the caller's perspective — this
+  ticket doesn't change the blocking nature of profile switching,
+  only which actor(s) the messages go to; see
+  [[0004-profile-switch-reindexing-cost]] for the separate question of
+  making that async).
+- Out of scope: the query fan-out layer for requests needing both raw
+  logs and profile-derived data in one response — that's
+  [[107-query-fan-out-across-primary-and-profile]]. Read-only query
+  handlers can be touched here only insofar as they need updating to
+  compile against the new handle shape, not to actually merge two data
+  sources yet.