Modules can report why they failed

A module had one channel back to its caller: the int returned from
dpm_module_execute, handed through by dpm_execute. Any detail behind
that number could only reach a log, so a caller wanting the reason had
to read output rather than ask for it.

dpm_set_last_error joins the exported API. A module records its reason
on the context handed to its entry point, which belongs to the caller,
and the caller reads it back with dpm_get_last_error.

The two accessors on the context now say what they do to it:
dpm_module_path becomes dpm_get_resolved_module_path, naming the value
it reports rather than the setting it came from, and dpm_last_error
becomes dpm_get_last_error, pairing with the setter.

Path normalization moves to sanitizers.cpp, which holds the conversions
that put a value written by a person into the single form the library
stores it in.

include/internal/context.hpp

@@ -89,5 +89,5 @@ namespace dpm_core {
      * @param ctx The libdpm-core.so context; NULL is a no-op
      * @param msg The failure description
      */
-    void set_error(dpm_ctx* ctx, const std::string& msg);
+    void set_last_error(dpm_ctx* ctx, const std::string& msg);
 } // namespace dpm_core

include/internal/sanitizers.hpp

@@ -0,0 +1,35 @@
+/**
+ * @file sanitizers.hpp
+ * @brief Normalizing a value into the shape the library stores it in
+ *
+ * @copyright Copyright (c) 2026 SILO GROUP LLC
+ * @author Chris Punches <chris.punches@silogroup.org>
+ *
+ * Part of the Dark Horse Linux Package Manager (DPM)
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as
+ * published by the Free Software Foundation, either version 3 of the
+ * License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program. If not, see <https://www.gnu.org/licenses/>.
+ */
+#pragma once
+
+#include <string>
+
+namespace dpm_core {
+    /**
+     * @brief Appends a trailing slash when one is absent
+     *
+     * @param s The path to normalize
+     * @return The path, ending in a slash unless it is empty
+     */
+    std::string with_trailing_slash(std::string s);
+} // namespace dpm_core