feat(orchestrator): C6 telemetry archive

[martinjms]

Implements #26 — orchestrator component C6 (operator-facing operational data persistence).

What this does

• `TelemetryArchive` subscribes to a `TelemetrySource` broadcast and writes each snapshot as a standalone JSON file under the configured dir.
• Filenames are `snapshot_<unix_ms>_.json` — monotonic + restart-safe (a new orchestrator process picks up where it left off without overwriting prior files because the `unix_ms` component keeps moving).
• `Uploader` trait abstracts S3 — tests use `MockUploader`; production wires a real S3 client (deferred; no `aws-sdk-s3` dep yet). `NoopUploader` is the local-only deployment fallback.
• Adds tokio `fs` + `io-util` features for async file I/O.

Out of scope (deliberate follow-up)

• A real S3 `Uploader` impl. The trait is wired and tests verify the call shape end-to-end; only the concrete S3 client is deferred until we add the aws-sdk dep + bucket config plumbing.

Test plan

• `cargo test -p arcane-swarm-orchestrator` — 56 passed, 0 failed (4 new C6 + 52 prior)
• `cargo clippy -p arcane-swarm-orchestrator --all-targets -- -D warnings` clean
• `cargo fmt --all -- --check` clean

Acceptance tests covered

All 4 from issue #26:

• `snapshots_written_locally_during_run`
• `snapshots_uploaded_to_s3` (verifies uploader received calls + bodies match local files byte-for-byte)
• `snapshot_schema_includes_required_fields` (timestamp + recent_commands + fleet + per-cluster stats)
• `restart_resumes_without_overwriting_prior_snapshots`

Closes #26.

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