refactor(logging): introduce consolidated public API

[VolkerChristian]

Summary

• add the single public <Log.h> API in snode::log with owned scope identity, severity/event/system-error logging, and startup settings
• migrate SNode.C production call sites to the public facade
• collapse duplicate backend output paths while retaining compatibility headers as migration surfaces
• document and test the public API

Verification

• complete SNode.C build with applications enabled
• PublicLogApiTest
• ParameterlessSemanticLoggerPolicyTest
• clean legacy LOG/VLOG/PLOG scan under src

[VolkerChristian]

Superseded by #262, which carries the same verified default-branch logging refactor on the combined integration branch.