Simplify startup logging paths and retire DefaultFactory runtime logger internals

[danielmarbach]

• Simplifies startup log routing by removing per-slot deferred state machinery (SlotFactoryState, per-logger deferred queues) in favor of slot-scoped startup buffering and deterministic draining on slot registration.
• Fixes a startup race where logs could fall back to the default logger during concurrent slot factory registration.
• Preserves startup visibility by flushing unresolved scoped startup logs to Trace when a slot is unregistered before its factory is available.
• Hollows out deprecated DefaultFactory runtime behavior and removes core DefaultLoggerFactory / DefaultLog path.
• Keeps legacy configuration bridge behavior for directory/log level via GetLoggingConfiguration().
• Slot-scoped startup logs stay slot-scoped and are drained when that slot factory registers.
• If a slot never gets a factory, pending scoped startup logs are emitted to Trace on unregister (single flush, no duplicates).
• Direct logger retrieval from DefaultFactory.GetLoggingFactory().GetLogger(...) is now explicitly unsupported and throws a meaningful exception.

Tradeoffs

• The Trace fallback only applies to a narrow edge case: logs written inside a slot scope before that slot ever gets a registered MEL-backed factory, followed by slot unregistration.
• In the normal path, endpoint startup wires MEL through DI and RegisterSlotFactory, so scoped startup logs are drained to the proper slot logger and never hit Trace.
• In other words, this is a last-resort safety net for abnormal startup/lifecycle failures, not a primary logging path.
• In the MEL/DI model, logger infrastructure is expected to exist once the host is built; pre-host or failed-host windows are inherently transitional, so a degraded fallback (Trace) is acceptable.
• This preserves signal (no silent loss of scoped startup logs) without reintroducing the removed legacy default logger pipeline.