Route project files through prj/<name>/ instead of flat at repo root

Repo root had ~35 groups of .prj/.dat files loose alongside the build
system and source, all sharing one flat namespace. Centralize the new
prj/<name>/ convention inside AStack::loadWeights/saveWeights/
loadTrainingBatch/saveTrainingBatch and StackCreator::fromFile/toFile
(via a new AStack::projectDir() helper) -- these already have access to
the project name, so callers (poet.cpp, the GUI) need no changes at all;
they keep passing the same base directory they always did.

Filenames inside each project folder are unchanged (e.g.
prj/mnist_2/mnist_2.prj, prj/mnist_2/mnist_2.Layer.0.w.dat) -- only the
directory moves. Added Matutils::ensureDir() (mkdir -p equivalent; no
std::filesystem in C++11) since ofstream won't create directories, used
wherever a project is saved for the first time.

File migration for existing projects is a separate commit.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016K8Gu7Qejd11JbdiHZqYAs

source/matutils.hpp

@@ -17,9 +17,32 @@
 #include "RnnTextHelper.hpp"
 
 #include <math.h>
+#include <string>
+#include <sys/stat.h>
 
 namespace Matutils
 {
+	// Creates every missing directory component of path (like `mkdir -p`).
+	// No std::filesystem in C++11, so this is a small POSIX-only helper.
+	inline void ensureDir(const std::string &path)
+	{
+		size_t pos = 0;
+		while (pos <= path.size())
+		{
+			size_t next = path.find('/', pos);
+			if (next == std::string::npos)
+			{
+				next = path.size();
+			}
+			std::string prefix = path.substr(0, next);
+			if (!prefix.empty())
+			{
+				mkdir(prefix.c_str(), 0755);
+			}
+			pos = next + 1;
+		}
+	}
+
 	const size_t NORMALIZING_DIM = 1;
 	inline arma::mat uniform(arma::mat const & sizeMat, double stdDev=1.0, double mu=0.5)
 	{