ENH: minor cleanup of memory streams and IOstreamOption

- make memory streams header-only (simpler)
- add sub-views and direct seek for span streams

New IOobject convenience methods

- IOobject::instanceValue() : return the IOobject instance as a scalar
  value (or 0). Effectively the same as instant(io.instance()).value()
  but with far less typing.

- IOobject::fileModificationChecking_masterOnly() : combines checks for
  time-stamp and inotify variants

STYLE: minor adjustments for Enum

applications/test/base64/Test-base64Encoding.cxx

@@ -0,0 +1,176 @@
+/*---------------------------------------------------------------------------*\
+  =========                 |
+  \\      /  F ield         | OpenFOAM: The Open Source CFD Toolbox
+   \\    /   O peration     |
+    \\  /    A nd           | www.openfoam.com
+     \\/     M anipulation  |
+-------------------------------------------------------------------------------
+    Copyright (C) 2016-2025 OpenCFD Ltd.
+-------------------------------------------------------------------------------
+License
+    This file is part of OpenFOAM.
+
+    OpenFOAM is free software: you can redistribute it and/or modify it
+    under the terms of the GNU General Public License as published by
+    the Free Software Foundation, either version 3 of the License, or
+    (at your option) any later version.
+
+    OpenFOAM 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 General Public License
+    for more details.
+
+    You should have received a copy of the GNU General Public License
+    along with OpenFOAM.  If not, see <http://www.gnu.org/licenses/>.
+
+Application
+    Test-base64Encoding
+
+Description
+    Test base64 encoding layer.
+
+    Target values generated with "base64 encode ..." in Google.
+    A simple independent source for comparison.
+
+\*---------------------------------------------------------------------------*/
+
+#include "base64Layer.H"
+#include "SpanStream.H"
+#include "List.H"
+#include "Pair.H"
+
+using namespace Foam;
+
+bool test(const Pair<string>& unit)
+{
+    const auto& input = unit.first();
+    const auto& expected = unit.second();
+
+    Foam::ocharstream os;
+
+    {
+        base64Layer b64(os);
+        b64.write(input.data(), input.size());
+
+        if (b64.close())
+        {
+            // Extra information
+            // std::cerr<< "closed with pending data" << nl;
+        }
+    }
+
+    const auto encoded = os.view();
+
+    Info<< input << nl;
+
+    if (encoded == expected)
+    {
+        Info<< "  encoded: " << encoded << " (OK)" << nl
+            << endl;
+        return true;
+    }
+    else
+    {
+        Info<< "  encoded: " << encoded << " (ERROR)" << nl
+            << "  expected: " << expected << nl
+            << endl;
+
+        return false;
+    }
+}
+
+
+bool test(std::initializer_list<Pair<string>> list)
+{
+    bool good = true;
+
+    for (const auto& t : list)
+    {
+        good = test(t) && good;
+    }
+
+    return good;
+}
+
+
+bool test(const UList<Pair<string>>& list)
+{
+    bool good = true;
+
+    for (const auto& t : list)
+    {
+        good = test(t) && good;
+    }
+
+    return good;
+}
+
+
+void testMixed(std::ostream& os, const UList<Pair<string>>& list)
+{
+    base64Layer b64(os);
+
+    os  << "<test-mixed>" << nl;
+
+    int i=0;
+    for (const auto& t : list)
+    {
+        const string& input = t.first();
+
+        os  << "<input" << ++i << " value='" << input << "'>" << nl;
+        os  << "  ";
+
+        b64.write(input.data(), input.size());
+        b64.close();
+
+        os  << nl
+            << "</input" << i << ">" << nl;
+    }
+
+    os  << "</test-mixed>" << nl
+        <<  nl;
+}
+
+
+// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
+
+int main(int argc, char * argv[])
+{
+    Info<< "Test base64 encode layer" << nl << endl;
+
+    List<Pair<string>> testList
+    {
+        {
+            "abcdef",  // 6 input, 8 output
+            "YWJjZGVm"
+        },
+        {
+            "OpenFOAM",
+            "T3BlbkZPQU0="
+        },
+        {
+            "OpenFOAM: The Open Source CFD Toolbox",
+            "T3BlbkZPQU06IFRoZSBPcGVuIFNvdXJjZSBDRkQgVG9vbGJveA=="
+        }
+    };
+
+
+    bool good = test(testList);
+
+
+    // Test mixing output
+    testMixed(std::cout, testList);
+
+    if (good)
+    {
+        Info<< "All tests passed" << endl;
+        return 0;
+    }
+    else
+    {
+        Info<< "One or more tests failed" << endl;
+        return 1;
+    }
+}
+
+// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //