Document the library's implementation

Every function in context.cpp, modules.cpp, and version.cpp carries a
Doxygen block, and the bodies explain the decisions the code alone does
not show: why dlerror is cleared before dlsym rather than testing the
returned address, what RTLD_NOW and RTLD_LOCAL buy, why missing contract
symbols are collected into one report, why a candidate that fails
validation is logged and skipped instead of failing the listing, and why
version parsing tests the first character itself.

Functions declared in a header carry a brief and implementation prose
rather than a second parameter list; Doxygen was reporting duplicate
documentation sections for each of them.

READMEs in include/internal and src/cli state what those directories
hold.

include/internal/README.md

@@ -0,0 +1,7 @@
+# Internal Headers
+
+libdpm-core.so's own headers, used by its sources and never installed.
+
+They declare the types and functions the library's translation units share with each other: the definitions behind the opaque handles `<dpm/core.h>` exposes, and the helpers those sources call across file boundaries.
+
+Nothing here is part of the public interface. The install rule ships `include/dpm/` alone, so a consumer or a module compiles against `<dpm/core.h>` and never sees this directory.