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/compact/compact/README.md

@@ -59,6 +59,10 @@ The `compact/*` events extend `SessionEventMap` (merge-extensible) via declarati
 
 Subclass `CompactService`, implement `compactIfNeeded` and `compactRegion`, and load the subclass as a plugin — it registers as `ctx.compact`. Every successful backend uses `COMPACT_CHECKPOINT_SOURCE` on its replacement user message; `isCompactCheckpointSource()` recognizes the marker after persistence or cloning without depending on backend identity. A template- or model-backed implementation can live as a sibling package without changing callers or the shared token meter.
 
+## Recognizing a checkpoint outside the host program (`./checkpoint`)
+
+`COMPACT_CHECKPOINT_SOURCE` and `isCompactCheckpointSource()` are declared on the `@deepseek-ai/dsh-compact/checkpoint` subpath and re-exported from the root, so host-side consumers keep reading them from the root. The leaf imports no cordis and declares no module augmentation (the [`dsh-commands/brand`](../../ui/commands/README.md) shape), which is what lets a client or wire program name the checkpoint source: the package **root** cannot enter such a program at all, because it reaches `dsh-session`'s root and that `Context` merge declares the host `sessions` service against the client's own (`TS2717` — one program per side, per [development.md](../../../docs/development.md#typescript-project-layout)). The web client's transcript adapter pins its plugin literal to this leaf with a type-only import, so renaming the plugin id here is a compile error there.
+
 ## Model Experience
 
 ### Conversation history, when a backend is invoked