Native acpmux chat pane for agent sessions (prototype)

[lawrencecchen]

Summary

Stacked on #15512 (bundles acpmux at Contents/Resources/bin/acpmux); merge that first.

Prototype. The agentSession panel now renders an acpmux session in a native AppKit chat pane instead of the WKWebView renderer. The pane talks JSON-RPC to the acpmux daemon over its Unix socket, starts the daemon when the socket does not answer, and streams the session into iMessage-style bubbles.

• Packages/macOS/CmuxAcpmux: newline-framed JSON-RPC over the acpmux socket, a detached daemon launcher (posix_spawn with setsid, readiness from the daemon log, no polling), Codable wire types, a pure TranscriptReducer that folds acpmux event records into rows, and the @Observable session model (local echo, reconnect with afterSeq, history paging, queue, permissions).
• Sources/Panels/AcpmuxChat: a virtualized NSTableView transcript with row heights cached by (row id, content version, width, group position, expansion), one diff per display frame through NSView.displayLink, bottom pinning with a "jump to latest" pill, scroll-anchored history paging, a send morph from the composer into the bubble, a typing indicator, collapsible tool activity, a permission card above the composer, queued prompts, and a session picker. Reduce Motion disables the animations.
• Tagged DEV builds use a private daemon home under the tag's Application Support directory and never touch ~/.acpmux.
• PanelType.agentSession and its snapshot keep their raw values; the snapshot adds the acpmux session id. cmux new-surface --type agent-session --provider claude opens the pane with that harness for new sessions.

The web renderer files stay in the target, unreferenced, for a later deletion commit.

Testing

Stacked on #15512; this branch merges its head d4bfdeb (SIGTERM shutdown, --ready-fd), and the bundled Contents/Resources/bin/acpmux comes from that head.

• Builds: fleet tagged builds acpchat through the normal path (cmux-ci build cmux --ref <sha>). Last one: job 033e39b4, head 7ab8791.
• Daemon launch: posix_spawn with an empty signal mask (POSIX_SPAWN_SETSIGMASK) and default dispositions (POSIX_SPAWN_SETSIGDEF), through sh -c 'set -m; "$@" &' so the shell does not set INT/QUIT to ignored. A spawned daemon shows BLOCKED 0 and ppid 1. Cause: Swift concurrency and dispatch worker threads run with most signals blocked, and the mask survives exec.
• Live SIGTERM during a running Codex turn: the daemon (pid from its ready line) exited in 0.18 s, its node and codex children were gone, and the pane reconnected and started a new bundled daemon. An idle daemon exited in 0.11 s.
• debug.window.record frame strips (60 fps, own window only), in artifacts/acpchat/round4/ of the worktree:
  • send morph: the flying bubble has its tail, and shape and color morph from the composer to the tailed bubble; sending bubbles are no longer dimmed, so the hand-off has no color pop;
  • send morph with a fast agent (test-only fake harness): the reply moves the slot mid-flight, and the morph now bends to the new slot instead of ending in two frames;
  • streaming growth: bubble and text grow in the same frame, with no frame that shows a line below the bubble;
  • permission card (fake harness, ask: prompt): slides in over about 4 frames above the composer.
• Scroll: 5,000-row synthetic transcript (debug.agent_chat.action seed_rows), scripted 3 s fling from bottom to top, frame time from the view's display link at 120 Hz (nominal 8.33 ms). Seeding takes 0.9 s. First fling (cold cells): p50 8.33 ms, p95 28.7 ms, p99 33.3 ms, max 35.3 ms, 107 dropped frames. Later flings: p50 8.33, p95 8.33, p99 8.3 to 18.5 ms, max 17.5 to 22.7 ms, 3 to 14 dropped frames. Before this round a first fling had a 2.0 s stall (all rows measured at once); unmeasured rows now get a character-count estimate and are measured in a 2 ms slice per frame.
• Test-only harness: a preflight script (outside the repo) adds a fake harness profile that runs cmux-tui/crates/acpmux/tests/fake_agent.py in the tagged daemon's config. No product code knows about it.
• Unit tests: swift test in Packages/macOS/CmuxAcpmux at 7ab8791: 27 tests in 5 suites passed. CI's swift-package-tests is skipped on this PR (the stacked Move acpmux into the cmux-tui workspace and bundle it in cmux.app #15512 changes workflow files, so change detection treats the diff as policy-only); it runs once Move acpmux into the cmux-tui workspace and bundle it in cmux.app #15512 merges.
• Localization: scripts/localization_catalog.py check passes.

Changelog

Changed: The agent session pane is a native chat view backed by acpmux, with streaming bubbles, tool activity, permission prompts, and a session picker.

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[teamleaderleo]

This remains a draft stacked on #15512; leaving it open until the acpmux bundle lands.