fix(tui,host): pin replayed compaction and correct projection wording

Review follow-ups on the append-origin transcript projection.
The live/replay equivalence claim was stated unconditionally but does not
cover `tool/call`: only replay re-derives call pairing, because a call
event carries no `surfaceOp` of its own and inherits transcript
membership from the `assistant/message` that advertised it — which the
live listener has necessarily just rendered. Narrow the claim in the TUI
README and Agent Note, and record at `rebuildTranscript` why the filter
is replay-only rather than a missing live branch.
Add `surface-replayed-compaction`: the three existing fixtures all come
from the live path, leaving the resume case the bug report leads with
pinned only by a unit test. The new checkpoint mounts with the
replacement already stored and records byte-identical to
`surface-after-compaction-wide`, so the two fixtures now pin the
equivalence they assert. The shared fixture appends move into
`appendPreCompactionLog` / `appendCompactionCheckpoint`.
`MESSAGE_TYPES` is not "human message event types" — it includes
`assistant/message`. Say what the code distinguishes (append-origin
conversation messages vs. model-only replacement copies) at the const,
the `paginate` and `session.history` JSDoc, the apiproxy README, and the
Agent Note.
Also: spell the replace shape as `Extract<SurfaceOp, { op: 'replace' }>`
for symmetry with the module's two other uses; document why
`isCompactCheckpoint` keeps a replacement check that is redundant at both
call sites; say that Ctrl+R toggles reasoning, which rebuilds the
transcript; and qualify "the sole source of derived history" as derived
*model* history now that the transcript is the other projection.

packages/ui/tui/tests/tui.snapshot.ts

@@ -51,6 +51,7 @@ const CHECKPOINTS = [
   'surface-before-compaction',
   'surface-after-compaction-narrow',
   'surface-after-compaction-wide',
+  'surface-replayed-compaction',
   'model-selector',
   'model-selector-filtered',
   'model-switching',
@@ -182,6 +183,64 @@ function appendToolResult(
   }, { surfaceOp: 'append' })
 }
 
+/** Frozen clock for the compaction fixtures; see the live scenario for why. */
+const COMPACTION_FIXTURE_TIME = new Date(2026, 6, 21, 14, 40, 0).getTime()
+
+/** The surface range a compaction checkpoint replaces, with its provenance. */
+interface CompactionRange {
+  start: number
+  end: number
+  sources: number[]
+}
+
+/**
+ * Append one prompt / tool-call / tool-result step, the history a compaction
+ * shadows on the model surface and the transcript must keep showing.
+ */
+function appendPreCompactionLog(session: Session): CompactionRange {
+  const user = session.append('user/message', createUserMessage({
+    content: [{ type: 'text', text: 'Old prompt with a long line that exercises wrapping and stays visible after compaction.' }],
+    source: { kind: 'user' },
+  }), { surfaceOp: 'append' })
+  const assistant = session.append('assistant/message', {
+    turn: 1,
+    step: 1,
+    message: createMessage({
+      role: 'assistant',
+      content: [{ type: 'tool-call', id: CallId('old-tool'), name: 'bash', arguments: '{}' }],
+      source: {
+        kind: 'model',
+        ...{ provider: 'mock', model: 'deepseek-v4-flash' },
+      },
+    }),
+  }, { surfaceOp: 'append' })
+  session.append('tool/call', { turn: 1, step: 1, callId: CallId('old-tool'), name: 'bash', arguments: '{}' })
+  const result = session.append('tool/result', {
+    turn: 1,
+    step: 1,
+    message: createToolResultMessage({
+      callId: CallId('old-tool'),
+      content: [{ type: 'text', text: 'tool output that stays readable after compaction' }],
+      isError: false,
+    }),
+  }, { surfaceOp: 'append' })
+  return { start: user.seq, end: result.seq, sources: [user.seq, assistant.seq, result.seq] }
+}
+
+/** Land a compaction: replace the range with the framed model-only checkpoint. */
+function appendCompactionCheckpoint(session: Session, range: CompactionRange): void {
+  session.append('user/message', createUserMessage({
+    content: [{
+      type: 'text',
+      text: '<context_checkpoint>\nModel-only summary payload that must never reach the transcript.\n</context_checkpoint>',
+    }],
+    source: COMPACT_CHECKPOINT_SOURCE,
+  }), {
+    surfaceOp: { op: 'replace', start: range.start, end: range.end },
+    sourceEventSeqs: range.sources,
+  })
+}
+
 function visualTool(
   name: string,
   call: NonNullable<ToolDefinition['presentCall']>,
@@ -689,57 +748,18 @@ describe('TUI terminal-state snapshots', () => {
     // Freeze the clock: the timing header hides zero-duration buckets, so a
     // real-clock millisecond tick between the fixture appends and the render
     // would flip `Tools 0.0s` in and out of the pinned header.
-    const nowSpy = vi.spyOn(Date, 'now').mockReturnValue(new Date(2026, 6, 21, 14, 40, 0).getTime())
-    let replacementStart = 0
-    let replacementEnd = 0
-    let replacementSources: number[] = []
+    const nowSpy = vi.spyOn(Date, 'now').mockReturnValue(COMPACTION_FIXTURE_TIME)
+    // beforeMount runs synchronously inside setupSnapshot, so the range the
+    // checkpoint replaces is assigned before the first await below.
+    let compacted!: CompactionRange
     const harness = await setupSnapshot({
       tools: ADVANCED_CARD_TOOLS,
-      beforeMount(session) {
-        const user = session.append('user/message', createUserMessage({
-          content: [{ type: 'text', text: 'Old prompt with a long line that exercises wrapping and stays visible after compaction.' }],
-          source: { kind: 'user' },
-        }), { surfaceOp: 'append' })
-        const assistant = session.append('assistant/message', {
-          turn: 1,
-          step: 1,
-          message: createMessage({
-            role: 'assistant',
-            content: [{ type: 'tool-call', id: CallId('old-tool'), name: 'bash', arguments: '{}' }],
-            source: {
-              kind: 'model',
-              ...{ provider: 'mock', model: 'deepseek-v4-flash' },
-            },
-          }),
-        }, { surfaceOp: 'append' })
-        session.append('tool/call', { turn: 1, step: 1, callId: CallId('old-tool'), name: 'bash', arguments: '{}' })
-        const result = session.append('tool/result', {
-          turn: 1,
-          step: 1,
-          message: createToolResultMessage({
-            callId: CallId('old-tool'),
-            content: [{ type: 'text', text: 'tool output that stays readable after compaction' }],
-            isError: false,
-          }),
-        }, { surfaceOp: 'append' })
-        replacementStart = user.seq
-        replacementEnd = result.seq
-        replacementSources = [user.seq, assistant.seq, result.seq]
-      },
+      beforeMount(session) { compacted = appendPreCompactionLog(session) },
     }, { columns: 80, rows: 24 })
     await checkpoint('surface-before-compaction', harness.terminal, { includeScrollback: true })
 
     await renderAfter(harness, () => {
-      harness.session.append('user/message', createUserMessage({
-        content: [{
-          type: 'text',
-          text: '<context_checkpoint>\nModel-only summary payload that must never reach the transcript.\n</context_checkpoint>',
-        }],
-        source: COMPACT_CHECKPOINT_SOURCE,
-      }), {
-        surfaceOp: { op: 'replace', start: replacementStart, end: replacementEnd },
-        sourceEventSeqs: replacementSources,
-      })
+      appendCompactionCheckpoint(harness.session, compacted)
       harness.terminal.resize(44, 18)
     })
     await checkpoint('surface-after-compaction-narrow', harness.terminal, { includeScrollback: true })
@@ -750,6 +770,23 @@ describe('TUI terminal-state snapshots', () => {
     nowSpy.mockRestore()
   })
 
+  // The resume path, which is what regressed for real users: the replacement is
+  // already stored when the terminal mounts, so the transcript comes from replay
+  // rather than from live appends. Pinned against the same log the live scenario
+  // ends on, at its wide size, so the two fixtures are directly comparable.
+  it('pins a stored compaction replayed at mount', async () => {
+    const nowSpy = vi.spyOn(Date, 'now').mockReturnValue(COMPACTION_FIXTURE_TIME)
+    const harness = await setupSnapshot({
+      tools: ADVANCED_CARD_TOOLS,
+      beforeMount(session) {
+        appendCompactionCheckpoint(session, appendPreCompactionLog(session))
+      },
+    }, { columns: 104, rows: 30 })
+    await checkpoint('surface-replayed-compaction', harness.terminal, { includeScrollback: true })
+    await disposeSnapshot(harness)
+    nowSpy.mockRestore()
+  })
+
   it('pins wrapped and explicit multiline shell-prompt input', async () => {
     const harness = await setupSnapshot({}, { columns: 44, rows: 18 })
     await renderAfter(harness, () => {