Finalize conversation and execution group documentation

docs/internal/observation.md

@@ -27,6 +27,8 @@ callback. The caller supplies normalized failure metadata; the recorder never
 parses an error string or receives provider bodies, credentials, headers, or
 other content. Events are values with no mutable containers. Callback panics
 propagate to the caller.
+The immediate parent run ID is set before start delivery and is retained as
+selection metadata changes. It describes ancestry, not implicit group membership.
 Resolved prompt and profile IDs are the exact runtime identities selected for
 the invocation, including namespace qualification when supplied by an engine
 view.
@@ -48,6 +50,13 @@ attempts above the first in a generation, and repair generations with at least
 one dispatch. A terminal event is an aggregate and must not be added to its
 attempt events.
 
+An execution group is a zero-ready synchronized collector of explicitly selected
+events. It enrolls by run ID and aggregates each run's first terminal counters and
+presence-aware usage exactly once. Snapshots copy members and totals under one
+lock; terminal coverage and token completeness are distinct. Observer fan-out
+copies the callback slice and invokes callbacks synchronously without holding a
+group lock. No group history, registry, or automatic nested membership exists.
+
 Smoke diagnostics mask inherited execution-recorder state while invoking
 generation or HTTP collaborators. Their private attempt evidence is assembled
 into a `SmokeTestReport`; it does not create ordinary run events or enter an