Add C++20 ownership and allocation core

Header-only primitives for the no-heap-after-startup design:
Result/Status, Region, ObjectPool with PoolPtr (std::unique_ptr with a
pool deleter), generation-checked Handle and asynchronous close support,
intrusive RefPtr, FixedBuffer, MonotonicArena, FixedHashMap and FixedLru,
IntrusiveList, and a bounded FixedHeap for third-party allocator hooks.

Unit tests cover each primitive (test_core_*). scripts/check_no_heap.py
rejects heap allocation in C++ sources under src/ and runs as a ctest.
clang-tidy now also analyses C++ sources and the core headers.

tests/test_core_arena.cpp

@@ -0,0 +1,63 @@
+/* SPDX-License-Identifier: GPL-3.0-or-later */
+#include <cstdint>
+#include <cstring>
+
+#include "core/arena.hpp"
+#include "test_helpers.hpp"
+
+namespace {
+
+struct Pair {
+    int a;
+    double b;
+};
+
+void test_allocate_alignment()
+{
+    ss::MonotonicArena arena;
+    SS_CHECK(arena.reserve(256));
+    auto c = arena.allocate(1, 1);
+    SS_CHECK(c);
+    auto d = arena.allocate(8, 64);
+    SS_CHECK(d);
+    SS_CHECK(reinterpret_cast<std::uintptr_t>(d->data()) % 64 == 0);
+    auto p = arena.make<Pair>(Pair{ 3, 1.5 });
+    SS_CHECK(p && (*p)->a == 3 && (*p)->b == 1.5);
+    SS_CHECK(!arena.allocate(8, 3));
+}
+
+void test_exhaustion_and_reset()
+{
+    ss::MonotonicArena arena;
+    SS_CHECK(!arena.allocate(1));
+    SS_CHECK(arena.reserve(64));
+    SS_CHECK(arena.allocate(64, 1));
+    auto over = arena.allocate(1, 1);
+    SS_CHECK(!over && over.error().code == ss::Errc::exhausted);
+    arena.reset();
+    SS_CHECK(arena.used() == 0 && arena.peak() == 64);
+    SS_CHECK(arena.allocate(32, 1));
+}
+
+void test_intern_and_arrays()
+{
+    ss::MonotonicArena arena;
+    SS_CHECK(arena.reserve(128));
+    auto s = arena.intern("aes-256-gcm");
+    SS_CHECK(s && *s == "aes-256-gcm");
+    SS_CHECK(s->data()[s->size()] == '\0');
+    auto empty = arena.intern("");
+    SS_CHECK(empty && empty->empty());
+    auto ports = arena.make_array<std::uint16_t>(4);
+    SS_CHECK(ports && ports->size() == 4 && (*ports)[3] == 0);
+}
+
+} // namespace
+
+int main()
+{
+    SS_RUN(test_allocate_alignment);
+    SS_RUN(test_exhaustion_and_reset);
+    SS_RUN(test_intern_and_arrays);
+    return 0;
+}