Move ACP snapshot replay doc into package

packages/ui/acp/snapshot-replay.md

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+
+# ACP Snapshot Replay
+
+This graph explains what a snapshot scenario proves: recorded real-model session logs are replayed keylessly, ACP stdout is normalized and diffed, and scenario workspaces preserve tool side effects that the UI stream alone cannot prove.
+
+```mermaid
+sequenceDiagram
+  participant Recorder as Real API recording
+  participant Fixture as snapshot fixture
+  participant Workspace
+  participant Replay as llm-replay adapter
+  participant ACP as acp-agent subprocess
+  participant Golden as stdout golden
+  Recorder->>Fixture: session.jsonl + workspace inputs
+  Fixture->>Workspace: seed files and hook configs
+  Fixture->>Replay: recorded StreamChunk script
+  Replay->>ACP: deterministic <code>llm/stream</code> chunks
+  ACP->>Workspace: bash, fs, and hook side effects
+  ACP->>Golden: normalized sessionUpdate stream
+  Golden-->>ACP: diff must be empty
+```
+
+The fs and hook snapshot matrix is valuable because it proves world state, hook decisions, and failed tool-card rendering, not just that replay returns text.
+
+Maintenance mode: curated Mermaid sequence based on the snapshot test harness.