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/tests/transcript-adapter.spec.ts

@@ -225,7 +225,7 @@ describe('TranscriptAdapter', () => {
 
     it.each([
       ['absent provenance', undefined],
-      ['malformed summary blocks', compactSummary(1, [{ type: 'image', data: 'nope' }])],
+      ['text-less summary blocks', compactSummary(1, [{ type: 'image', data: 'nope' }])],
       ['a whitespace-only summary', compactSummary(1, [{ type: 'text', text: '   ' }])],
       ['an empty summary array', compactSummary(1, [])],
       ['a non-array summary', compactSummary(1, 'plain string')],
@@ -240,6 +240,19 @@ describe('TranscriptAdapter', () => {
       ])
     })
 
+    it('keeps the text of a mixed-block summary, skipping the blocks it cannot render', () => {
+      // ContentBlock is merge-extensible and the payload type is ContentBlock[],
+      // so a non-text block must not discard recoverable text beside it.
+      const adapter = new TranscriptAdapter()
+      adapter.reset([
+        compactSummary(1, [{ type: 'text', text: '可用摘要' }, { type: 'image', data: 'nope' }]),
+        checkpoint(2, 1, { start: 0, end: 0, sourceEventSeqs: [1, 0] }),
+      ])
+      expect(adapter.nodes()).toEqual([
+        { kind: 'compaction', seq: 2, time: 1_700_000_000_002, summary: '可用摘要' },
+      ])
+    })
+
     it('leaves the summary null when the checkpoint records no provenance at all', () => {
       const adapter = new TranscriptAdapter()
       adapter.reset([at(2, {