Fix clean target, correct build docs, add overview and docs guide

clean was deleting the generated build system along with the artifacts —
CMakeCache.txt, the makefiles, CMakeFiles/, the CMake file API directory,
and the CTest configuration. That left the build directory unconfigured
after every clean, so an IDE reading its target list from the file API
lost every target until the project was reloaded. clean now removes only
what the build produced.

Also writes CTest's scratch directory into DartConfiguration.tcl so a bare
ctest and IDE test discovery use it too, not just build-driven runs.

BUILD.md no longer lists libdl as a dependency; glibc 2.34 merged it into
libc, so nothing links against it. The code-reference section moves to its
own DOCUMENTATION.md.

The fixture config logged at ERROR while the info module emits its output
at INFO, so the CLI invocation BUILD.md documents exited 0 and printed
nothing. The fixture now logs at INFO and the documented command works as
written.

OVERVIEW.md describes what DPM is, how core routes and validates modules,
and why the architecture takes the shape it does.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

tests/fixtures/conf/core.conf

@@ -1,5 +1,5 @@
 [logging]
-log_level = ERROR
+log_level = INFO
 write_to_log = false
 
 [modules]