dashboard: get rid of oatpp

[MariusBgm]

Drop oat++ from the SIL Kit Dashboard client

• removed: ThirdParty/oatpp submodule, its CMake and license entries
• added: minimal HTTP/1.1 client over the bundled standalone asio (dashboard/http/)
• changed: dashboard JSON now emitted with the bundled rapidyaml
• changed: wire format identical bar 3 cosmetics: space after :, no \/, UTF-8 over \uXXXX; unescapable control chars become U+FFFD
• fixed: registry hung on shutdown when the dashboard accepted but never answered; requests now have connect/write/read deadlines and abort after a grace period
• fixed: use-after-free, RegistryInstance destroyed the dashboard before the registry pointing at it
• fixed: one unmappable event killed the worker thread, silently ending all dashboard reporting; failures are now logged and skipped per event and per flush
• fixed: unhandled controller types threw instead of being skipped, reachable by 11 of 21 types
• removed: bulk-update probe, which killed the worker on a negative result; also drops a startup round-trip and a ~7 s stall when unreachable
• fixed: events pending at shutdown were discarded, now flushed
• fixed: dashboard log lines had no Dashboard topic and escaped topic filtering
• changed: SILKIT_BUILD_DASHBOARD=OFF with --dashboard-uri now reports the missing feature plainly
• added: warning when the dashboard is on but CollectFromRemote is off, which silently yields no metrics or attributes
• changed: SilKitEvent type erasure to std::variant, worker split out; net −348 lines
• tests: new suites for HTTP client, parser, retry, JSON writer, DTO mapper, worker, shutdown; IRestClient mocked, batching had no coverage
• tests: no sleeps or wall-clock assertions, latch handover, stable under CI load