Summary

• Adds a new all-smi snapshot subcommand that produces a one-shot, machine-readable dump of hardware state (GPU/CPU/memory/chassis/process/storage) to stdout or a file and exits — targeted at scripting, CI probes, Slurm prolog/epilog hooks, and jq/awk pipelines.
• Supports three output formats: JSON (default, pretty when stdout is a TTY), CSV with --query dot-path column selection, and Prometheus exposition that byte-for-byte reuses the existing src/api/metrics/* exporters for parity with /metrics.
• Re-exports SnapshotOptions, Snapshot, SnapshotError, SnapshotHardFailure, run, run_with_collector, and the SnapshotCollector trait from all_smi for programmatic use and test doubles.

Implementation notes

• Module layout — src/snapshot/{mod.rs, options.rs, collector.rs, query.rs, serializers/{json,csv,prometheus}.rs}. All files under 400 lines per the project's soft limit.
• Prometheus reuse — the Prometheus serializer calls GpuMetricExporter, CpuMetricExporter, MemoryMetricExporter, ChassisMetricExporter, DiskMetricExporter, ProcessMetricExporter, NpuMetricExporter, and HardwareMetricExporter directly, in the same order as api::handlers::metrics_handler. An integration test asserts byte-for-byte containment of the GPU exporter output inside the snapshot output.
• Timeouts — each reader call runs inside tokio::task::spawn_blocking wrapped in tokio::time::timeout(opts.timeout_per_reader, ...). On timeout, a SnapshotError { kind: "timeout", ... } is appended and the snapshot continues; on join panic, kind: "panic". Library semantics are not changed.
• Hard failure vs soft — when every sample collects zero devices across every section, run_with_collector returns Err(SnapshotHardFailure). main.rs maps this to exit code 1. Partial failures (some devices collected, some sections errored) produce exit code 0 with the errors embedded in the errors array.
• TTY detection — uses std::io::IsTerminal (Rust 1.70+) to auto-resolve --pretty when not explicitly set. Pretty is on when stdout is a TTY, off when piped.
• Chassis power aggregation — mirrors api::server::run_api_mode: when GPUs report power and chassis does not, the GPU power sum backfills total_power_watts so snapshot Prometheus output stays consistent with a /metrics scrape on the same hardware.
• Absent vs empty — requested-but-empty sections serialize as empty arrays ([]), NOT-requested sections are omitted entirely from the JSON object, per the spec.
• CSV column set — --query is tokenised, trimmed, de-duplicated while preserving order, and each cell is resolved via the dot-path evaluator over serde_json::Value. Missing paths yield empty cells (never panic). RFC-4180 quoting is applied inline.
• Cargo deps — moved anyhow from the mock feature to cli so the snapshot subcommand can use anyhow::Context and anyhow::Error::new for the library entry point. No new external crates.
• Pre-existing clippy fixes — the pre-commit hook auto-applied cargo clippy --fix to src/network/metrics_parser.rs (collapsible-match warnings introduced by a newer rustc). Included in this commit to keep the working tree clean; they are functionally equivalent to the pre-commit code.

Testing

Verified on Linux (NVIDIA GB10 / Spark SBC) with --all-features disabled (system lacks clang headers for furiosa-smi-rs). Tested:

• all-smi snapshot emits valid JSON with schema, timestamp, and at least one device array.
• all-smi snapshot --format csv --query index,name,utilization,temperature emits the expected 4-column CSV (with --include gpu for GPU-only rows).
• all-smi snapshot --format prometheus reuses the exporter byte-for-byte (integration test).
• all-smi snapshot --include cpu,memory omits gpus, chassis as absent JSON keys.
• all-smi snapshot --samples 3 --interval 1 emits a JSON array of 3 timestamped objects roughly 1s apart.
• all-smi snapshot --output /tmp/x.json writes the file and prints nothing to stdout.
• Hard failure path exits 1; flag parse error (invalid --include) exits 2.
• tests/snapshot_test.rs — 9 integration tests covering JSON, CSV, Prometheus, hard-failure, missing-paths, absent-sections, and stdout-dash routing, all passing.
• Unit tests: 29 new tests across snapshot::{mod, options, serializers::{json, csv, prometheus}, query}. All 576 library tests green.
• cargo build, cargo fmt --all -- --check green. cargo clippy clean for the snapshot module (pre-existing warnings in unrelated files are untouched beyond the hook-applied clippy-fix noted above).

Not covered in this PR

• Windows / macOS / non-Linux GPU platforms — I only had a Linux NVIDIA system to test on. The snapshot subcommand uses the same reader factories as api mode, so it inherits platform support from the existing code; but the platform matrix item from the acceptance criteria is left unchecked pending CI verification on those runners.

Closes #185