core: O_NOFOLLOW the download target open

DAEMON's safepath-adversarial.md accepts a TOCTOU residual between its
canonicalise-and-check and the download starting, on the stated grounds
that CORE's O_NOFOLLOW open of the final file closes it. That flag was
never actually set: SparseFile::open used O_WRONLY|O_CREAT|O_CLOEXEC, so
a symlink swapped in as the final path component after DAEMON's check
would be followed and redirect our pwrites outside the allowed roots.

Add O_NOFOLLOW. A symlinked leaf now fails the open with ELOOP, which
errno_to_error already maps to Error::path_rejected. Regular files and
the O_CREAT of a fresh part file are unaffected; resume (existing regular
part file) is unaffected. Test that a symlinked destination is rejected
rather than silently followed, and that the link target is never touched.

Co-Authored-By: Claude Sonnet 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01HPPSGhiArbvQgwC2DNiURS

core/src/io/sparse_file.cpp

@@ -74,7 +74,12 @@ Result<void> SparseFile::open(std::string_view path, const OpenOptions &opts) {
         return ErrorInfo(Error::internal, "SparseFile already open");
 
     std::string p(path);
-    int flags = O_WRONLY | O_CREAT | O_CLOEXEC;
+    // O_NOFOLLOW: the final component of a download target must never be a symlink, on
+    // create or on resume. DAEMON canonicalises the path and checks it against the allowed
+    // roots before start(), but a symlink swapped in afterwards would redirect our writes
+    // outside those roots (daemon/docs/safepath-adversarial.md leans on this open closing
+    // that TOCTOU window). A symlinked leaf fails here with ELOOP -> Error::path_rejected.
+    int flags = O_WRONLY | O_CREAT | O_CLOEXEC | O_NOFOLLOW;
     if (opts.truncate_existing)
         flags |= O_TRUNC;