round 1: project the web transcript from append-origin events

Replace the surface-ordered fold with a log-ordered human transcript:
append-origin surface events at their own log positions plus one marker per
landed compaction checkpoint. Command folding, the tool-call index, and the
rev-keyed memo carry over unchanged.

Removes foldDegraded, the padding sentinels, baseSeq, and degradedSeqs() --
they existed only to satisfy the core fold's seq === index assertion. That
also closes the pagination hole A1 exposed: a page can carry a checkpoint
whose shadowed range fell outside the window, and nothing resolves
surfaceOp.start anymore.

apps/web/tests/snapshots/seeded-history/ui.expected.md

@@ -36,7 +36,11 @@
   - img
 - button "在新对话中分支":
   - img
-- text: {{clock}} cache hit 98% · 15,962 tokens · 1 turns · 2 steps
+- text: {{clock}}
+- button "上下文已压缩 点击查看压缩摘要":
+  - img
+  - text: 上下文已压缩 点击查看压缩摘要
+- text: cache hit 98% · 15,962 tokens · 1 turns · 2 steps
 - textbox "Message the agent"
 - button "Add attachment":
   - img