btc: save graph_db stat log on clean shutdown (LTC-parity site 3/3)

[frstrtr]

What

src/c2pool/main_btc.cpp stands up the graph_db stat-log with only 2 of LTC's 3 persistence sites:

• Site 1 — load-on-start: main_btc.cpp:1722 mi->load_stat_log();
• Site 2 — periodic-100s: main_btc.cpp:1735 mi->save_stat_log();
• Site 3 — save-on-shutdown: absent (LTC has it at main_ltc.cpp:7456-7457).

Effect: everything accumulated in the final sub-100s window is dropped on every clean restart. The dashboard series only survives when a periodic tick happens to land before shutdown. Same 2/3 gap DGB fixed in #1097.

Change

One null-guarded call added after the run loop returns (the while (!shutdown_initiated) run_for-slice loop), while web_server and its MiningInterface are still alive, before the graceful-teardown resets:

if (web_server) {
    if (auto* mi = web_server->get_mining_interface())
        mi->save_stat_log();
}

Mirrors #1097 / dgb 822eaa347 exactly, adapted to BTC line numbers.

Isolation

src/c2pool/main_btc.cpp only — BTC's own entrypoint. Zero src/core, zero cross-coin (no main_ltc.cpp / main_dgb.cpp touched). p2pool-merged-v36 surface: NONE (operator dashboard stat log, not share/PPLNS/coinbase bytes).

Evidence

(live restart-persist proof captured below — tick lands, clean restart, series survives the shutdown boundary)