feat(orchestrator): C3 cluster /stats collector

[martinjms]

Implements #23 — orchestrator component C3. Fills in the `unimplemented!()` bodies on the `StatsCollector` scaffold landed in PR #40 and unignores the acceptance tests.

What this does

• `poll_once` — fetches each endpoint sequentially, stores a `StatsSample`, prunes by the sample's own `sampled_at` against the retention window so the rolling-window semantic holds even under bursty polling. Failed fetches don't store a sample and don't abort the round.
• `current_rates` — computes `bytes_out_per_sec` by diffing oldest/newest in each per-cluster series against `sampled_at`; `egress_aggregate_gbps` is the sum across clusters in Gbps. `delta_hit_rate` is a proxy (until the cluster exposes `broadcasts_attempted` per #94).
• `time_series_for` — returns the per-endpoint series clone.
• `run` — simple periodic poll loop driven by `tokio::time::interval`. Never returns under normal operation; production wraps in `tokio::spawn`.

Test plan

• `cargo test -p arcane-swarm-orchestrator` — 48 passed, 0 failed (3 new C3 + 45 prior)
• `cargo clippy -p arcane-swarm-orchestrator --all-targets -- -D warnings` clean
• `cargo fmt --all -- --check` clean

Acceptance tests covered

All 3 from issue #23:

• `collector_reports_correct_rates_and_counters`
• `polling_continues_when_cluster_unreachable`
• `time_series_memory_bounded`

Closes #23.

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