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.

tsconfig.base.json

@@ -54,6 +54,7 @@
       "@deepseek-ai/dsh-llm/brand": ["./packages/llm/llm/src/brand.ts"],
       "@deepseek-ai/dsh-llm/message": ["./packages/llm/llm/src/message.ts"],
       "@deepseek-ai/dsh-commands/brand": ["./packages/ui/commands/src/brand.ts"],
+      "@deepseek-ai/dsh-compact/checkpoint": ["./packages/compact/compact/src/checkpoint.ts"],
       "@deepseek-ai/dsh-tools/presentation": ["./packages/core/tools/src/presentation.ts"],
       "@deepseek-ai/dsh-user-approval/types": ["./packages/ui/user-approval/src/types.ts"],
       "@deepseek-ai/dsh-user-interaction/types": ["./packages/ui/user-interaction/src/types.ts"],