benchd M0 — the skeleton: cargo workspace, BENCH_SUITE isolation, socket, event log, CLI exit codes, and its own gate

[Wirasm]

First milestone of the bench build. Read docs/bench-roadmap.md before anything else — it is written for a session with no prior context, and its invariants section is binding. Rationale and decisions live in docs/workbench-audit-2026-08.md. The name bench/benchd is settled (roadmap, naming note); do not reopen it.

Scope — one complete, green, provable M0 slice

• daemon/ cargo workspace at the repo root, per the tree in the roadmap's M0 section (decided, not up for redesign): crates benchd, bench, bench-wire under daemon/crates/. No root Cargo.toml — cargo invoked at the repo root must fail loudly, not half-work.
• BENCH_SUITE=<name> isolation from day one: socket path, record directory, and all state move together; refusals are loud (port the semantics of helm's HELM_DEFAULTS_SUITE / DefaultsSuite, including refusing rather than silently falling back to the live instance). Unset = the canonical paths.
• benchd binds a Unix socket at the suite-aware path; bench talks to it.
• Append-only event log (JSONL) and the record layout under ~/.bench/ (suite-aware).
• bench CLI skeleton: --suite, and the exit-code discipline ported from the spool scripts — 0 ok, 2 no daemon, 3 refused (with a reason on stderr), 4 daemon accepted and failed.
• Port helm's request-id newtype rule into bench-wire (helm landed it as The spool request id is a bare String, and the gate that makes it a filename is asked at two of six sites #260 — port, don't reinvent: an ungated id must be unrepresentable at any path builder).
• daemon/AGENTS.md: the gate command, conventions, and a pointer to the roadmap's invariants.
• daemon/test.sh gate: cargo fmt --check, clippy, cargo test, plus a conformance harness that runs the real bench binary as a subprocess and checks both directions (the SpoolWireConformanceTests shape — stand up the pattern now, even while the surface is small).
• CI: a separate job triggered on daemon/** changes. The Swift gate must keep needing only the Swift toolchain.

Prove

• Two instances (live + a BENCH_SUITE instance) run side by side with zero shared state — and the negative control: the suite instance's state directories and socket never touch the canonical paths.
• The conformance harness fails when an exit code or wire shape drifts (demonstrate once by breaking it, per the watch-it-fail rule).

Out of scope

Taps, attention queue, mail, spawning, ptys — all later milestones. Do not touch Sources/, hooks/, pi/, or tools/. Nothing is unwired in M0.