fix: CWD restore, install error checks, fetch EINTR

- build.cpp: save/restore CWD on both success and failure paths
- install.cpp: check error_code after each filesystem op, abort on failure
- install.cpp: check write_installed() return value
- fetch.cpp: EINTR retry loop on both waitpid calls (exec_cmd + exec_capture)

src/fetch/fetch.cpp

@@ -2,6 +2,7 @@
 #include "kappa/paths.hpp"
 
 #include <array>
+#include <cerrno>
 #include <cstdio>
 #include <cstdlib>
 #include <filesystem>
@@ -29,7 +30,8 @@ static int exec_cmd(const std::vector<std::string>& argv) {
     }
     if (pid < 0) { return -1; }
     int status = 0;
-    waitpid(pid, &status, 0);
+    pid_t w;
+    do { w = waitpid(pid, &status, 0); } while (w == -1 && errno == EINTR);
     return WIFEXITED(status) ? WEXITSTATUS(status) : -1;
 }
 
@@ -61,7 +63,8 @@ static std::string exec_capture(const std::vector<std::string>& argv) {
         result += buf.data();
     }
     close(pipefd[0]);
-    waitpid(pid, nullptr, 0);
+    pid_t w;
+    do { w = waitpid(pid, nullptr, 0); } while (w == -1 && errno == EINTR);
 
     if (!result.empty() && result.back() == '\n') { result.pop_back(); }
     return result;