feat(init): macOS launchd + shellrc fallback + --start-daemon

[jiunbae]

Summary

Closes the v0.4.1 UX gap where muxa init finished cleanly on macOS but muxa status failed because nothing had ever started muxad. Three additive pieces, no breaking changes:

• muxad-launchd — macOS analogue of muxad-systemd. muxa init writes ~/Library/LaunchAgents/dev.open330.muxad.plist, runs launchctl bootstrap gui/<uid> + kickstart -k. Uninstall does the inverse. Same outcome / dry-run / marker semantics as the systemd path.
• muxad-shellrc — cross-platform fallback for hosts with no service manager (containers, BSDs, WSL1, minimal Linux, CI). Marker block in ~/.zshrc / ~/.bashrc / fish config / fallback ~/.profile, lazy-starting muxad from the next interactive shell. Shell pick is auto from $SHELL.
• --start-daemon flag (default true) — final action that spawns muxad detached if pgrep -x muxad doesn't find it. Belt-and-suspenders alongside the manager components: if systemd's enable --now or launchd's bootstrap silently no-ops, the explicit start still leaves muxad responsive when the wizard finishes.

Preset::Standard / Preset::Full and the wizard's multi-select pick exactly one of the three managers from std::env::consts::OS. Detection::recommended_daemon_manager further degrades to shellrc when systemctl/launchctl is missing on a host that'd normally support them — the "Standard preset = working install" invariant holds even in stripped-down envs.

Why now

Reported by a user on macOS (after the v0.4.1 fix landed): muxa init succeeded but muxa status returned daemon not reachable at /tmp/muxa-501.sock. Pre-flight pgrep matched a stale process; IPC socket wasn't bound. muxad-launchd is the macOS-native fix; muxad-shellrc + --start-daemon close the gap on every other systemd-less platform too.

Module layout

File	Role
files/launchd.rs (new, ~120 LOC)	macOS plist content + launchctl bootstrap/bootout driver
files/shellrc.rs (new, ~120 LOC)	shell rc marker block, $SHELL-aware path selection
components.rs	MuxadLaunchd / MuxadShellrc variants + recommended_daemon_manager() + applicable_here() filter
detect.rs	launchctl_available field + recommended_daemon_manager() host degrader
plan.rs	EnableLaunchdUnit { plist_path } / DisableLaunchdUnit / StartDaemonIfNeeded actions; plan_systemd / plan_launchd / plan_shellrc helpers
apply.rs	New action handlers + detached muxad spawner; refactored apply_one into named helpers (was at clippy's line cap)
mod.rs	--start-daemon flag + appender; new step lines
ui.rs	Multi-select filtered to applicable_here() only

Test plan

• cargo test --workspace — 365 / 365 pass (was 361 + 4 new)
• cargo clippy --workspace --all-targets -- -D warnings — clean
• cargo fmt --all -- --check — clean
• Linux dry-run: muxa init --dry-run --preset standard --yes → systemd, no launchd, no shellrc; start muxad if not running line appears
• macOS smoke: muxa init --preset standard --yes on a real macOS host — confirms plist written, agent loaded, muxa status responsive without manual intervention
• Container / WSL1 smoke: muxa init --preset standard --yes → falls back to muxad-shellrc, daemon starts
• Round-trip: muxa init --preset standard --yes then muxa init --uninstall leaves a clean state

Release

Ship as v0.4.2 (minor — additive new feature).

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