Move metric declarations and bucket computation to the profile actor (ticket 104)

Metric declare/update/remove, backfill, and per-line apply_all_metrics
now run on the profile actor (engine::profile) instead of inline in the
primary's ingest closure, per idea 0001's resolved decision that the
primary handles params only. The profile actor reuses EngineSnapshot as
its own snapshot type -- declare_metric/update_metric/apply_all_metrics
work unchanged -- and mirrors the primary's param declarations into its
own snapshot on every wake (and synchronously before handling a
DeclareMetric/UpdateMetric message, since there's no ordering guarantee
between the watch-wake and message branches in tokio::select!) so
metric label/value resolution keeps working without storing a second
copy of raw log text; backfill reads lines through its cloned primary
IndexReader instead.

EngineHandle gained a profile_reader field so MetricDeclarations/
BucketValues/Promql queries route to the profile actor's snapshot
instead of the primary's -- without it, metrics could be declared but
never queried, since the primary no longer computes or stores them.
DeclareMetric/UpdateMetric/RemoveMetricDeclaration messages are still
sent to the primary's mpsc channel and proxied to the profile actor by
handle.rs's run() loop; EngineHandle's write-side API is otherwise
unchanged (ticket 106 gives API handlers a direct route). Alert
evaluation still reads the primary's (now metric-less) snapshot
directly -- an accepted gap until ticket 105 moves it, per idea 0001's
resolved decision that alerts move with metric computation.

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

@@ -1,73 +0,0 @@
-# 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.