Document Ubuntu 24 daemon build

doc/build-ubuntu-24.md

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+Copyright (c) 2026 Agrarian Developers
+
+Ubuntu 24.04 Daemon Build Notes
+================================
+
+These notes describe the native daemon build path tested against current Ubuntu
+24.04 system packages.
+
+Recommended host size
+---------------------
+
+Use at least 2 GB RAM for a reliable build. A 512 MB host with 2 GB swap can
+configure successfully, but compilation is extremely slow and may time out while
+building large translation units.
+
+Packages
+--------
+
+Install the native build dependencies:
+
+    sudo apt-get update
+    sudo apt-get install -y \
+      build-essential pkg-config autoconf automake libtool bsdmainutils \
+      libboost-all-dev libevent-dev libgmp-dev libssl-dev \
+      libdb5.3-dev libdb5.3++-dev
+
+Daemon-only build
+-----------------
+
+For the pool daemon, the GUI, tests, bench, ZMQ, and UPnP can be disabled:
+
+    ./autogen.sh
+    ./configure \
+      --without-gui \
+      --disable-tests \
+      --disable-bench \
+      --disable-zmq \
+      --with-miniupnpc=no \
+      --with-incompatible-bdb \
+      CXXFLAGS="-O0 -g0 --param ggc-min-expand=1 --param ggc-min-heapsize=32768"
+    make -j1
+
+OpenSSL 3
+---------
+
+Ubuntu 24.04 ships OpenSSL 3. Agrarian no longer rejects this only because
+`RAND_egd` is unavailable. Some deprecated OpenSSL SHA calls still warn during
+compilation; those warnings are expected until the hashing helper code is
+modernized.
+
+Berkeley DB
+-----------
+
+Ubuntu 24.04 provides Berkeley DB 5.3. Wallet builds with BDB 5.3 are not
+portable back to BDB 4.8 wallet environments, so only use wallets created by this
+build with compatible BDB 5.3 builds.