Adds streamViaAcp: a drop-in alternative to streamViaCli that drives Claude
through the claude-code-cli-acp bridge over ACP instead of `claude -p`. Returns
the same AssistantMessageEventStream, so streamSimple dispatches to either
transport behind a kill-switch (FUSION_CLAUDE_ACP=1 + an injected bridge path),
OFF by default — the live `-p` path is byte-for-byte untouched until soak.
- Full-history prompt every turn (buildPrompt) — the ACP path has no --resume (R13).
- Forwards schema-only MCP servers so Claude emits correct tool calls; breaks
early on the first tool_call (cancel turn, surface to pi) so the bridge never
executes Fusion's tools — mirrors the `-p` break-early pattern.
- Translation reuses the tested createEventBridge by synthesizing Claude stream
events from ACP session/updates, sharing pi sequencing + tool-name mapping.
- Bridge env forwards only HOME/PATH so `claude` authenticates from the login
session (R17); never inherited process.env or API keys.
Verified: 3/3 translation unit tests; real-bridge session/update shapes confirmed
(agent_message_chunk text + tool_call); 326/326 existing pi-claude-cli tests green;
typecheck clean.
Remaining for Route A: engine injection of the bridge path (KTD10), U12 picker/
auth/status, U13 workflow verification.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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