Define the documentation build in docs/CMakeLists.txt and collect the prose

The Doxygen configuration, the page list, and the docs target live in
docs/, and the written documents live in docs/PROSE/, leaving the
top-level file with the output paths and the add_subdirectory call.

The subdirectory is given its own binary directory so that CMake's
scaffolding stays out of the documentation output, which holds pdf/ and
html/ and nothing else.

Paths both files need are set once above the call: the child's output
and cleanup and the parent's clean rule refer to the same variables.

docs/PROSE/BUILD.md

@@ -0,0 +1,88 @@
+# Building DPM Core and CLI {#build}
+
+## Prerequisites
+
+- GCC/G++ supporting C++20
+- CMake 3.22 or later
+- Make
+- Doxygen (optional, for the code reference)
+- `pdflatex` and `makeindex` (optional, for the PDF code reference)
+
+The library itself depends only on libc and libstdc++, so it builds and runs on a minimal system.
+
+## Development Builds
+
+Configure a build tree and compile it:
+
+```
+cmake -B <build-dir> -DCMAKE_BUILD_TYPE=Debug
+cmake --build <build-dir>
+```
+
+Artifacts land in:
+
+```
+<build-dir>/bin/dpm             the dpm binary
+<build-dir>/lib/libdpm-core.so  the library
+<build-dir>/modules/info.so     the bundled info module
+```
+
+The `dpm` binary built here finds the locally built `libdpm-core.so` on its own — an embedded library search path points it at the `lib/` directory next to it in the build tree, so running it directly uses the library you just built with nothing to set up first. This is only the default: `LD_LIBRARY_PATH` takes precedence over the embedded path, so the binary can be pointed at any other libdpm-core, including the system-installed one.
+
+### Automated Testing
+
+```
+ctest --test-dir <build-dir> --output-on-failure
+```
+
+This runs the API test binary (which exercises the full load-time validation matrix against the fixture modules in `tests/fixtures/`) and the CLI end-to-end tests.
+
+### Manual Execution
+
+The build tree plus the test fixtures form a complete self-contained environment; no installation is required. Point the `dpm` binary at local paths with its override flags:
+
+```
+<build-dir>/bin/dpm --config-dir ./tests/fixtures/conf --module-path <build-dir>/modules info version
+```
+
+The flags `--config-dir`, `--module-path`, `--root`, and `--log-level` each redirect the corresponding system default; `--root` sets the target root that package-operation modules act on.
+
+## Release Builds
+
+One build tree carries the whole sequence, so the artifacts that get tested are the artifacts that get installed:
+
+```
+cmake -B <build-dir> -DCMAKE_BUILD_TYPE=Release -DCMAKE_INSTALL_PREFIX=/usr
+cmake --build <build-dir>
+ctest --test-dir <build-dir> --output-on-failure
+cmake --install <build-dir>
+```
+
+This is the packaging flow: configure once, build once, test what was built, install what was tested.
+
+### Automated Testing
+
+The test suite builds and runs in any configuration, so a release tree is tested by the same command a development tree is:
+
+```
+ctest --test-dir <build-dir> --output-on-failure
+```
+
+Running it between the build and the install step is what makes the sequence above a packaging flow rather than four unrelated commands.
+
+### Installation
+
+```
+cmake --install <build-dir>
+```
+
+Omit `-DCMAKE_INSTALL_PREFIX=/usr` at configure time for a `/usr/local` install. Under the install prefix this installs:
+
+```
+bin/dpm                   the dpm binary
+lib/libdpm-core.so        the library
+lib/dpm/modules/info.so   the bundled info module
+include/dpm/              the public header
+```
+
+The library's own configuration installs to `/etc/dpm/conf.d/core.conf` regardless of prefix.