round 2: pin checkpoint recognition at compile time

Move COMPACT_CHECKPOINT_SOURCE and isCompactCheckpointSource into a
cordis-free src/checkpoint.ts leaf, re-exported from the root so every
host-side consumer keeps its import. The client can then type-import the
leaf without reaching dsh-session's root, whose Context merge declares the
host sessions service and collides with the client's -- the dsh-commands/brand
shape. Renaming the plugin id now fails the client typecheck.

Also: keep recoverable summary text when a compact/summary mixes text with
other block types, and capture the seeded-history provenance seqs from the
pushes that produce them instead of deriving them by arithmetic.

packages/client/runtime/src/client/sessions/conversation.ts

@@ -129,7 +129,14 @@ export interface CompactionSummaryNode {
   summary: string | null
 }
 
-/** Fallback for surface events this UI version does not know. */
+/**
+ * Fallback for surface events this UI version does not know: the documented
+ * default arm of `SessionEventMap`, which is merge-extensible, so the
+ * projection's switch cannot end in `assertNever`. No event produces this node
+ * today — `isAppendSurfaceEvent` admits only the four types in core's
+ * `SurfaceEventType`, and each has its own arm — and it exists so widening that
+ * set core-side degrades to a raw row instead of dropping the event silently.
+ */
 export interface UnknownSurfaceNode {
   kind: 'unknown'
   seq: number