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/host/apiproxy/src/api-proxy.ts

@@ -58,17 +58,18 @@ import { openNativePath } from './native-path-opener.ts'
 /** Page size when history is called without maxMessages. */
 const DEFAULT_MAX_MESSAGES = 50
 
-/** Human message event types (the pagination counting unit). */
+/** Conversation message event types (the pagination counting unit). */
 const MESSAGE_TYPES = new Set(['user/message', 'assistant/message', 'steering/message'])
 
 /**
- * Message-boundary pagination: count maxMessages append-origin human messages
- * backwards from the window tail. Replacement copies are model-only, so they
- * consume no quota; the page stays one contiguous raw range, which keeps a
- * compaction's log-only provenance on the same page as its replacement. The cut
- * is the starting seq of the oldest message group (chunks group via
- * sourceEventSeqs — never cut mid-message). The tail page naturally includes the
- * in-progress partial.
+ * Message-boundary pagination: count maxMessages append-origin messages
+ * backwards from the window tail. Replacement copies never entered the
+ * conversation a reader sees — they restate a shadowed range for the model
+ * alone — so they consume no quota; the page stays one contiguous raw range,
+ * which keeps a compaction's log-only provenance on the same page as its
+ * replacement. The cut is the starting seq of the oldest message group (chunks
+ * group via sourceEventSeqs — never cut mid-message). The tail page naturally
+ * includes the in-progress partial.
  */
 function paginate(
   events: readonly SessionEvent[],