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/package.json

@@ -15,12 +15,17 @@
       "types": "./lib/types/invariant.d.ts",
       "default": "./lib/invariant.js"
     },
+    "./checkpoint": {
+      "types": "./lib/types/checkpoint.d.ts",
+      "default": "./lib/types/checkpoint.js"
+    },
     "./src/*": "./src/*",
     "./package.json": "./package.json"
   },
   "files": [
     "lib/index.js",
     "lib/invariant.js",
+    "lib/types/**/*.js",
     "lib/types/**/*.d.ts",
     "lib/types/**/*.d.ts.map",
     "src"