Stand up the profile actor skeleton and primary->profile watch signaling (ticket 103)

Adds a second actor task with its own left_right-backed ProfileSnapshot
and a tokio::sync::watch the primary's IndexWriter bumps after every
publish, so the profile actor wakes and re-reads newly-visible primary
lines through its own cloned IndexReader instead of polling. No metric
computation, alert evaluation, or query routing moves yet -- this is
pure plumbing, proven with a test that ingests a line on the primary
and observes the profile actor's watermark advance in response.

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

tickets/103-profile-actor-skeleton-and-primary-signaling.md

@@ -1,67 +0,0 @@
-# 103 — Stand up the profile actor skeleton and primary→profile signaling, with no behavior moved yet
-
-## Summary
-
-First slice of [[0001-actor-split]]'s step 3 (splitting the single
-engine actor into reader/primary/profile actors). This ticket is pure
-plumbing: a second actor task with its own `left_right` write side for
-profile-owned state, a way for it to read the primary's index
-(`EngineHandle`'s `IndexReader` is already `Clone`, so no primary-side
-change is needed to hand out a read-only view), and a
-`tokio::sync::watch` the primary signals on after each `left_right`
-sync so the profile actor wakes without busy-polling. No metric
-computation, alert evaluation, or query routing moves in this ticket —
-that's [[104-move-metric-computation-to-profile-actor]] and
-[[105-move-alert-evaluation-to-profile-actor]]. Depends on
-[[101-extract-metric-computation-into-separate-stage]] (done) for the
-`ingest_params`/`apply_all_metrics` seam this split will eventually
-route across two actors.
-
-## Where
-
-- `src/engine/handle.rs:33-47` (`IndexWriter`) — `mutate()` calls
-  `self.handle.append(...)` then `self.handle.publish()`; this is
-  where the new watch-sender notification hooks in, right after
-  `publish()`.
-- `src/engine/handle.rs:48-73` (`IndexReadHandle`/`IndexReader`) — the
-  existing read-side pattern to mirror for the profile actor's own
-  derived state; `IndexReader` is already `Clone`
-  (`handle.rs:61-67` is a template, not something to reuse directly —
-  the profile actor needs its own `left_right` pair for its own
-  state, plus a cloned primary `IndexReader` for read access).
-- `src/engine/handle.rs:74-78` (`EngineHandle`) and `:119-124`
-  (`spawn`) — the shape to parallel for a new profile-actor spawn
-  function.
-- `src/engine/snapshot/mod.rs:208-255` (`logs_range`/
-  `logs_in_sequence_range`) — the read-side primary API the profile
-  actor will use (via its cloned `IndexReader`) to pull newly-visible
-  lines after waking on the watch signal.
-
-## Scope
-
-- Add a `ProfileSnapshot` type (new module, e.g.
-  `src/engine/profile.rs`) with its own `left_right` write/read pair,
-  parallel to `EngineSnapshot`'s pattern in `handle.rs` — for now it
-  can be effectively empty (a placeholder struct), since no state
-  moves into it yet.
-- Add a `tokio::sync::watch<u64>` (carrying, e.g., the primary's
-  latest processed sequence) that `IndexWriter::mutate` sends on after
-  `publish()` succeeds.
-- Add a profile actor task: holds a cloned primary `IndexReader` plus
-  its own `ProfileSnapshot` write side, wakes on the watch channel,
-  and — for this ticket only — reads whatever new lines
-  `logs_in_sequence_range` exposes since its last-seen sequence and
-  does nothing with them (e.g. just advances its own watermark). The
-  goal is proving the wake/read loop works end-to-end, not doing real
-  work with the data yet.
-- Wire `spawn` (`src/cli/mod.rs:95`) to also spawn the profile actor
-  alongside the primary, but do not yet change what `ApiState` holds
-  or what any API handler talks to — `EngineHandle` and its public API
-  stay exactly as they are today. That's
-  [[106-split-enginehandle-api-for-primary-and-profile]]'s concern.
-- Out of scope: moving `apply_all_metrics`, metric declarations, or
-  alert evaluation; changing `EngineHandle`'s public surface; eviction
-  coordination beyond what already exists (primary still evicts on its
-  own schedule per [[0001-actor-split]]'s resolved decision — this
-  ticket doesn't need to handle the profile actor falling behind
-  eviction yet, since it isn't consuming anything meaningful).