Update documentation for Agrarian 2.0 modernization

doc/modernization.md

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+Agrarian 2.0 Modernization Status
+=================================
+
+This branch is the modernization line for Agrarian. The goal is to preserve
+network behavior while making the codebase build repeatably on current systems.
+
+Verified Build Baseline
+-----------------------
+
+Use Ubuntu 24.04 as the primary build host.
+
+The current deterministic `depends/` baseline is:
+
+- Qt 6.8.3 LTS
+- OpenSSL 3.5.6 LTS
+- Boost 1.91.0
+- Berkeley DB 4.8.30 for portable legacy wallet compatibility
+- protobuf 2.6.1 for the existing Qt payment request sources
+- libevent 2.1.8-stable
+- GMP 6.1.2
+- ZeroMQ 4.3.1 when enabled
+
+The currently verified binaries are:
+
+- Ubuntu daemon and command-line tools
+- Ubuntu Qt desktop wallet
+- Windows x86_64 Qt desktop wallet and command-line tools cross-compiled from
+  Ubuntu
+
+Recommended Commands
+--------------------
+
+Native Ubuntu daemon:
+
+    JOBS=1 ./contrib/build-linux.sh
+
+Native Ubuntu desktop wallet:
+
+    JOBS=1 ./contrib/build-linux-wallet.sh
+
+Windows x86_64 desktop wallet:
+
+    JOBS=1 ./contrib/build-win64-wallet.sh
+
+Use higher `JOBS` values only on hosts with enough memory. On an 8-core, 16 GB
+host, `JOBS=8` is reasonable.
+
+Verification
+------------
+
+After a native Ubuntu build, run:
+
+    ./contrib/smoke-test-daemon.sh
+    ./contrib/smoke-test-wallet.sh
+    ./contrib/smoke-test-qt.sh
+
+After a Windows cross build, confirm the expected PE binaries:
+
+    file src/qt/agrarian-qt.exe src/agrariand.exe src/agrarian-cli.exe src/agrarian-tx.exe
+
+Documentation Status
+--------------------
+
+The preferred docs are:
+
+- `doc/build-ubuntu-24.md`
+- `doc/build-wallets.md`
+- `doc/build-windows.md`
+- `doc/dependencies.md`
+
+Older macOS, RPM, Gitian, and Travis documents remain in the tree as historical
+reference. They are not part of the verified 2.0 modernization path unless a
+document explicitly says otherwise.