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/STYLE.md

@@ -0,0 +1,60 @@
+# Code Style Guide {#style}
+
+Allman, with four modifications.
+
+## Base
+
+Allman: each brace on its own line, and the body indented one level inside them.
+
+## Modification: Opening Braces Stay on the Line That Opens the Scope
+
+An opening brace does not move to a line of its own. It stays on the line of the function definition, control statement, namespace, `extern "C"` block, struct, or enum that opens the scope. The closing brace keeps its own line.
+
+## Modification: Every Scope Indents
+
+Allman indents the body of a scope, and here that applies to every scope without exception — namespaces and `extern "C"` blocks included, not only functions and control statements. One level is four spaces.
+
+```
+namespace dpm_core {
+    bool parse_version(const char* s, long out[3]) {
+        if (!s || !*s) {
+            return false;
+        }
+        return true;
+    }
+} // namespace dpm_core
+```
+
+## Modification: `} else {` Stays on One Line
+
+Allman would put the closing brace, the `else`, and the opening brace on three lines. They stay on one.
+
+```
+if (level <= DPM_LOG_WARN) {
+    std::fprintf(stderr, "%s: %s\n", level_name(level), message);
+} else {
+    std::fprintf(stdout, "%s\n", message);
+}
+```
+
+## Modification: A Short Guarded Statement May Occupy One Line
+
+Where a condition guards a single short statement, the braces and the statement stay on the line with the condition. The braces are still written.
+
+```
+if (v == "FATAL") { return DPM_LOG_FATAL; }
+if (v == "ERROR") { return DPM_LOG_ERROR; }
+if (v == "WARN")  { return DPM_LOG_WARN;  }
+```
+
+## Modification: A Closing Brace Names the Scope It Ends
+
+Where the opening line is far enough above to be off screen — a namespace or an `extern "C"` block — the closing brace carries a comment naming what it closes.
+
+```
+} // namespace dpm_core
+```
+
+```
+} /* extern "C" */
+```