wallet: define CHDChain's static constants out of line

hush-gtest failed to link with "undefined reference to
CHDChain::VERSION_HD_MNEMONIC". The version constants are static const int with
in-class initialisers and no definition anywhere, so any ODR use needs one --
and gtest's EXPECT_*/ASSERT_* macros take their arguments by const reference,
which is exactly that. test_mnemonic_compat.cpp:161 passes VERSION_HD_MNEMONIC
to EXPECT_LT.

dragonxd links either way, because nothing in the daemon binds these to a
reference; only the test target exposed it, and the test target was never built
on the branch that introduced the test.

Define all four rather than only the one that failed: VERSION_HD_BASE,
VERSION_HD_TRANSPARENT and CURRENT_VERSION carry the identical latent fault, and
the next EXPECT_EQ against any of them would hit the same wall. Fixing the test
instead would have hidden the problem rather than removed it.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/wallet/walletdb.cpp

@@ -34,6 +34,15 @@
 #include <boost/scoped_ptr.hpp>
 #include <boost/thread.hpp>
 
+// Out-of-line definitions for CHDChain's in-class static constants. These are
+// only initialised in the class body, so any ODR use -- binding one to a const
+// reference, which is exactly what gtest's EXPECT_*/ASSERT_* macros do -- needs
+// a definition or the link fails. hush-gtest hit this on VERSION_HD_MNEMONIC.
+const int CHDChain::VERSION_HD_BASE;
+const int CHDChain::VERSION_HD_TRANSPARENT;
+const int CHDChain::VERSION_HD_MNEMONIC;
+const int CHDChain::CURRENT_VERSION;
+
 using namespace std;
 
 static uint64_t nAccountingEntryNumber = 0;