introduce Accumulator interface in statsengine to separate ingestion from snapshot-building

Define statsengine.Accumulator (Ingest + Reset) to represent the
event-accumulation responsibility separately from runtime.SnapshotSource
(Snapshot), which handles the read side. This reduces the SRP violation in
Engine: callers that only push events now hold an Accumulator; callers that only
read statistics hold a SnapshotSource.

- Add Accumulator interface and compile-time assertion in statsengine/engine.go
- Add EventIngester type alias (= statsengine.Accumulator) in runtime/runtime.go
  with a compile-time assertion, so callers in the runtime layer can reference
  the ingestion contract without importing statsengine directly
- Split tuiRuntime.engine field into accumulator + snapSource so the event-loop
  callback holds Accumulator and wireRuntimeBindings passes SnapshotSource to
  SetDashboardSnapshotSource — making each consumer's dependency explicit
- Simplify resetDashboardSnapshotSource in tui.go to cast for interface{ Reset() }
  independently of Snapshot(), removing the combined ad-hoc interface check

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

1 files changed, 14 insertions, 2 deletions

diff --git a/internal/runtime/runtime.go b/internal/runtime/runtime.go
index e8e278e..5a3c9a9 100644
--- a/internal/runtime/runtime.go
+++ b/internal/runtime/runtime.go
@@ -40,10 +40,20 @@ type EventSink interface {
 // behind this interface so the dashboard can retrieve live snapshots.
 // Snapshot returns (nil, nil) when the engine is nil. A non-nil error
 // indicates that snapshot construction failed and the result must be discarded.
+// This is the read side of the stats engine; the write side is
+// statsengine.Accumulator.
 type SnapshotSource interface {
 	Snapshot() (*statsengine.Snapshot, error)
 }
 
+// EventIngester is the write-only, event-feeding side of the stats engine,
+// as needed by the trace event loop. It is an alias for the statsengine.Accumulator
+// contract so callers in the runtime layer can reference a single type without
+// importing statsengine directly. Callers that only push events should hold an
+// EventIngester; callers that only read statistics should hold a SnapshotSource.
+// *statsengine.Engine satisfies both interfaces.
+type EventIngester = statsengine.Accumulator
+
 // Snapshotter is the read-only subset of the trie contract used by consumers
 // that only need to poll the version and retrieve snapshot data. It mirrors the
 // Snapshotter interface in internal/tui/flamegraph but lives here so the core
@@ -208,9 +218,11 @@ var (
 	// *probemanager.Manager must satisfy the probe-control surface exposed to the TUI.
 	_ ProbeManager = (*probemanager.Manager)(nil)
 
-	// *statsengine.Engine must satisfy the snapshot-source contract used by the
-	// dashboard and the TUI runtime.
+	// *statsengine.Engine must satisfy both the snapshot-source contract (read
+	// side) and the event-ingestion contract (write side). These interfaces
+	// represent the two distinct responsibilities of the engine.
 	_ SnapshotSource = (*statsengine.Engine)(nil)
+	_ EventIngester  = (*statsengine.Engine)(nil)
 
 	// *streamrow.RingBuffer must satisfy the full event-sink contract (read +
 	// write sides), which is a superset of StreamSource.