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/104-move-metric-computation-to-profile-actor.md

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+# 104 — Move metric declarations and bucket computation into the profile actor
+
+## Summary
+
+Second slice of the split: metric declarations, `apply_all_metrics`,
+and the declare/update/remove/backfill machinery move off the primary
+`EngineSnapshot` and onto the profile actor's `ProfileSnapshot`
+(stood up in [[103-profile-actor-skeleton-and-primary-signaling]]).
+The primary keeps raw log storage and param declarations only, per
+[[0001-actor-split]]'s resolved decision that the primary "extracts
+declared + auto params only... no metrics, no profile logic." The
+profile actor consumes newly-visible primary lines (via the
+watch-wake loop from ticket 103) and runs metric regexes against them
+itself, building its own bucket data independent of the primary.
+
+## Where
+
+- `src/engine/snapshot/mod.rs:364-434` (`validate_metric_request`,
+  `declare_metric`, `resolve_backfill_and_store_metric`) and
+  `:434-487` (`validate_update_metric`, `update_metric`,
+  `remove_metric_declaration`) — the declare/update/remove machinery
+  to move onto `ProfileSnapshot`.
+- `src/engine/snapshot/mod.rs:587-617` (`backfill_metric`,
+  `apply_all_metrics`) — backfill replays `self.logs_range()` (a
+  primary-side accessor) through `apply_metric_to_log`; once metrics
+  live on the profile actor, backfill needs to read logs through the
+  profile actor's cloned primary `IndexReader` instead of `self`.
+- `src/engine/handle.rs` — `EngineMessage::DeclareMetric`/
+  `UpdateMetric`/`RemoveMetricDeclaration` arms (see the message list
+  at `src/engine/message.rs:30-63`) currently route through the
+  single actor's `write.mutate` against `EngineSnapshot`; these need
+  to route to the profile actor's `ProfileSnapshot` instead.
+- `src/engine/handle.rs`'s `Ingest` arm — `snap.apply_all_metrics(...)`
+  is called directly after `ingest_params` today
+  (see ticket 101's change); this call moves out of the primary's
+  ingest closure entirely once the profile actor computes metrics
+  itself from the lines it observes via the watch/read loop, rather
+  than the primary computing them inline.
+
+## Scope
+
+- Move `MetricDeclaration`/bucket-related `IndexKey`/`DataValue`
+  variants' *usage* (not necessarily their type definitions, which are
+  shared) onto `ProfileSnapshot`'s own index/map, separate from the
+  primary's.
+- Move `declare_metric`/`update_metric`/`remove_metric_declaration`/
+  `backfill_metric`/`apply_all_metrics` (and their validation
+  counterparts) to operate on `ProfileSnapshot`, reading raw log text
+  via the profile actor's cloned primary `IndexReader` wherever they
+  need `logs_range`-style access, instead of `self.index` on the
+  primary.
+- Remove the `apply_all_metrics` call from the primary's `Ingest` arm
+  in `handle.rs` — the primary no longer computes metrics inline; the
+  profile actor's watch-wake loop (from ticket 103) now calls
+  `apply_all_metrics`-equivalent logic itself per newly-visible line.
+- `DeclareMetric`/`UpdateMetric`/`RemoveMetricDeclaration` messages
+  need a new route to the profile actor (a second `mpsc` channel, or
+  folding into the profile actor's own message enum) — decide the
+  concrete mechanism during implementation; `EngineHandle`'s public
+  API doesn't need to change yet if this ticket keeps routing internal
+  to `handle.rs` (full API surface changes are
+  [[106-split-enginehandle-api-for-primary-and-profile]]'s scope).
+- Out of scope: alert evaluation (still reads the primary's snapshot
+  directly today — [[105-move-alert-evaluation-to-profile-actor]]
+  moves it), and the profile-switch (`activate_profile`) reload path
+  (still targets whatever this ticket lands as the metric-owning
+  actor, but retargeting `src/api/mod.rs:812-852` explicitly is
+  [[106-split-enginehandle-api-for-primary-and-profile]]'s scope).
+- Backfill cost and blocking behavior: per [[0001-actor-split]]'s
+  performance rationale, this move is what stops a slow backfill from
+  blocking the primary's ingest/query-serving — confirm with a test
+  that ingesting new lines on the primary keeps working while the
+  profile actor is mid-backfill on a newly declared metric.