feat(orchestrator): C5 telemetry SSE source + dashboard server

[martinjms]

Implements #25 — orchestrator component C5 (read-side observability plane).

What this does

Two new modules:

• `telemetry.rs` — `TelemetrySource` builds `TelemetrySnapshot` from live state (`DriverPool` snapshot + `CommandDispatcher` log slice + `StatsCollector` per-cluster latest), broadcasts to subscribers via `tokio::broadcast`.
• `sse_server.rs` — minimal HTTP/SSE server (no new deps; raw TCP + HTTP/1.1) on `GET /telemetry/stream`. Per-connection task subscribes and writes `data: \n\n` events.

Wire format uses serializable types (no `Instant` on the wire). Internal `Instant` values are converted to ms-age fields relative to snapshot time so subscribers don't depend on the orchestrator's monotonic clock.

Supporting changes

• `DriverPool::snapshot()` — returns `Vec` so the source can embed fleet state
• `StatsCollector::latest_per_cluster()` — returns `HashMap<String, ClusterStats>` for per-cluster wire summary

Out of scope (deliberate follow-up)

• The binary `orchestrator-cli` that consumes the SSE stream and renders a terminal dashboard. The acceptance test for "CLI connects, renders, reconnects" is exercised by a generic SSE client (raw `TcpStream` + HTTP/1.1) that verifies the server-side reconnect tolerance — the CLI binary lands when there's a real terminal UX to ship.

Test plan

• `cargo test -p arcane-swarm-orchestrator` — 52 passed, 0 failed (4 new C5 + 48 prior)
• `cargo clippy -p arcane-swarm-orchestrator --all-targets -- -D warnings` clean
• `cargo fmt --all -- --check` clean

Acceptance tests covered

All 4 from issue #25:

• `sse_stream_emits_valid_json_events` (raw HTTP/SSE client parses `data:` lines, deserializes `TelemetrySnapshot`)
• `cli_connects_renders_and_reconnects` (drop + reconnect via fresh TcpStream)
• `multiple_subscribers_each_receive_events` (two concurrent SSE clients)
• `stream_continues_across_command_activity` (submit a command, verify it appears in the next snapshot's `recent_commands`)

Closes #25.

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