Bounty #1600: Add RustChain stats dashboard

[MrHyplex9511]

Closes rustchain-bounties #1600.

Adds a standalone RustChain Stats Dashboard.

Deliverable

• explorer/stats-dashboard.html — zero-dependency SPA (vanilla JS, no build step). Shows live Network status, Epoch (# + slot), Active Miners (online/total), Epoch Pot, Total Supply (8,388,608 RTC), Transactions, Architecture breakdown bars, and a Top Miners leaderboard. Auto-refreshes every 10 s. Mobile-responsive grid + tables. Empty/offline states degrade gracefully. All server-rendered values escaped (no innerHTML with raw data) to avoid XSS.
• explorer/stats_server.py — stdlib-only Python server (no Flask/requests). Serves the dashboard and proxies an allow-listed set of node endpoints (health, epoch, api/miners, api/transactions, api/stats) with CORS. Contains offline self-tests: python3 stats_server.py --test.

Run

cd explorer
python3 stats_server.py            # http://localhost:9090
python3 stats_server.py --node http://localhost:8000
python3 stats_server.py --test     # offline unit tests

Or serve statically: python3 -m http.server 9090.

Test

7/7 unit tests pass (endpoint allow-list, proxy fetch against fake upstream, HTML served with expected title).