fix(tui,host): project the human transcript from append-origin events

The terminal and history pagination both treated the model-visible surface as
the human transcript. A landed compaction replacement therefore erased the
conversation it summarized — messages the reader had already seen — and a
model-only replacement copy consumed a page's `maxMessages` quota, which could
also split a compaction's provenance from the replacement citing it.

`dsh-session` now exports the marker split `isAppendSurfaceEvent` /
`isReplacementSurfaceEvent`. The terminal replays append-origin surface events,
keeps a shadowed step's tool cards paired through its append-origin assistant
message, and renders one dim marker where a compaction landed; the checkpoint is
recognized through the compaction seam's `isCompactCheckpointSource` contract,
not the shape of the replacement. `session.history` counts only append-origin
human messages. Everything model-facing keeps reading `session.surface`.

packages/ui/tui/tests/snapshots/surface-after-compaction-narrow.expected.txt

@@ -1,7 +1,7 @@
-terminal 44x18 buffer=normal length=18 base=0 viewport=0
+terminal 44x18 buffer=normal length=24 base=6 viewport=6
 lifecycle started=1 stopped=0 progress=inactive
 title "DSH snapshot"
-cursor hidden column=7 viewportRow=14 bufferRow=14
+cursor hidden column=7 viewportRow=17 bufferRow=23
 buffer
 0| " DEEPSEEK HARNESS"
   style 1-8 fg=bright-magenta bold
@@ -13,25 +13,39 @@ buffer
 3| <blank>
 4| "Assistant                                   "
   style 0-8 fg=bright-magenta bold underline
-5| "Model wait 0.0s                             "
+5| <blank>
+6| "You                                         "
+  style 0-2 fg=bright-magenta bold underline
+7| "Old prompt with a long line that exercises  "
+8| "wrapping and stays visible after compaction."
+9| <blank>
+10| "● Tool / bash / Run the coverage gate"
+  style 0-36 fg=green
+11| "$ pnpm run test:coverage                    "
+  style 0-23 dim
+12| "/workspace/project                          "
+  style 0-17 dim
+13| "packages/ui/tui 100%                        "
+  style 0-19 dim
+14| "… +1 lines (Ctrl+O to expand)               "
+  style 0-28 dim
+15| "1 test skipped                              "
+  style 0-13 dim
+16| "coverage complete                           "
+  style 0-16 dim
+17| "[exit 0]                                    "
+  style 0-7 dim
+18| "Model wait 0.0s                             "
   style 0-14 dim
-6| <blank>
-7| "Context · workspace-context"
-  style 0-26 dim
-8| "Additional instructions from:               "
-  style 0-43 dim
-9| "nested/AGENTS.md                            "
-  style 0-15 dim
-10| "                                            "
-11| "Render workspace context XML clearly.       "
-  style 0-36 dim
-12| <blank>
-13| "/workspace/project (tui-staging)  deepseek-v"
+19| <blank>
+20| "… earlier context was compacted …           "
+  style 0-32 dim
+21| <blank>
+22| "/workspace/project (tui-staging)  deepseek-v"
   style 0-17 fg=bright-magenta bold
   style 18-31 dim
   style 34-43 dim
-14| " dsh >                                      "
+23| " dsh >                                      "
   style 1-3 fg=bright-magenta bold
   style 5-6 dim
   style 7-7 inverse
-15-17| <blank>