MXD v0.2.6

End-to-end smoke test of letsgo testnet on a clean Ubuntu 24.04 VM
exposed two distribution bugs:

1. letsgo apt prereq was hardcoded librocksdb6.11 (the 22.04
   package name). Ubuntu 24.04 ships librocksdb8.9, Debian 12 ships
   librocksdb7.8, etc. Apt install failed with Unable to locate package librocksdb6.11.
2. mxd_node crashed with a rocksdb assertion (SaveError: errptr != nullptr) on Ubuntu 24.04. libmxd's chain code passes NULL for
   the rocksdb char** errptr parameter, which rocksdb 6.x tolerates
   (mainnet runs 22.04/6.11) but rocksdb 8.x asserts-aborts.

v0.2.6 fixes the distribution surface so both problems go away without
touching the chain code. The libmxd-side NULL-errptr issue is real and
worth fixing eventually, but it doesn't affect operators using these
bundles.

Changed

• Build matrix pinned to ubuntu-22.04 (and ubuntu-22.04-arm).
  glibc 2.35 baseline — forward-compatible with 24.04, Debian 12,
  Fedora 36+, Rocky 9+, Arch (rolling), Alpine 3.18+ (with gcompat).
  Also matches the mainnet validators' OS so the shipped binary is
  byte-compatible with production.
• Bundle is now self-contained via recursive ldd traversal in
  the Stage release bundle step. Every dynamic dependency of
  mxd_node and libmxd.so is bundled into lib/ — including
  libssl, libcrypto, libsodium, librocksdb (6.11 matching
  mainnet, no 8.x assert issue), libcurl and its sub-deps
  (libnghttp2, libidn2, librtmp, libssh, libpsl,
  libgssapi_krb5, libldap, liblber), libminiupnpc,
  libmicrohttpd, libgmp, libcjson, libzstd, on top of the
  source-built libuv, libuvwasi, libm3, liboqs. Glibc
  components (libc, libm, libpthread, libdl, librt,
  ld-linux, vdso) are intentionally excluded — they're kernel-
  coupled and must come from the host.
• SONAME symlinks reconstructed in the bundle. cp -L flattens
  the symlink chain, so the bundle step rebuilds
  libfoo.so → libfoo.so.X → libfoo.so.X.Y.Z so loaders find each
  lib by its SONAME at runtime.
• letsgo no longer runs apt-get install. The bundle has every
  .so it needs; the wrapper's LD_LIBRARY_PATH=bundle/lib wins over
  the system's /usr/lib/.../. Removes the entire class of "apt
  package name doesn't exist on this distro" bugs.
• Bundle README simplified to a 2-step quick-start (no apt line).

Notes

• Tarball size grows from ~3 MB → estimated ~30–50 MB compressed
  (~15 extra .so files). Trade-off accepted for distro portability.
• Operators on Ubuntu 22.04 will see the bundled libs slightly
  duplicated against system libs, but the bundle ones win — no
  conflict, just a few MB of disk.
• Mainnet validators continue to use the source-built path (via
  install_dependencies.sh + letsgo --from-source or direct
  cmake) and are unaffected by the bundle changes.

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