Configuration and logging move out of the context source

context.cpp had grown to hold three unrelated concerns: the context
lifecycle, everything about the .conf file format, and everything about
writing a log message.

conf.cpp now owns the format end to end - parsing a file into the store,
reading values back out, and interpreting a configured string as the
boolean or log level a setting holds. logging.cpp owns the write path:
the level filter, the stream choice, and the name a level carries in
output.

context.cpp keeps what a context is: resolving each setting from the
override, the configured value, or the built-in default, and reporting
what it resolved.

CMakeLists.txt

@@ -21,7 +21,9 @@ set(CMAKE_CXX_STANDARD_REQUIRED ON)
 # libdpm-core.so — the library (C ABI)
 # ---------------------------------------------------------------------
 add_library(dpm-core SHARED
+        src/core/conf.cpp
         src/core/context.cpp
+        src/core/logging.cpp
         src/core/modules.cpp
         src/core/version.cpp
 )