Fix wrong impl failing IPv4-in-IPv6 when IPv4-mapped IPv6 is missing

---
Fix wrong root in MMDB tree parser, which failed IPv4 lookups in IPv6
databases that don't include IPv4-mapped IPv6 addresses.

A bug that was part of locus since forever but didn't show up in MaxMind
databases since those appear to always include the IPv4-mapped IPv6
addresses node.

Thanks to https://github.com/nickjacob who left me the clues I needed in
an earlier closed PR:
* #39

CHANGELOG.md

@@ -4,6 +4,13 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/)
 and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.html).
 
+## [Unreleased]
+
+### Fixed
+
+- wrong root in MMDB tree parser, failing IPv4 lookups in IPv6 databases that
+  don't include IPv4-mapped IPv6 addresses [wrong since 1.0.0]
+
 ## [2.3.7] - 2024-03-16
 
 ### Added

src/locus_mmdb_tree.erl

@@ -45,8 +45,12 @@
 %% Macro Definitions
 %% ------------------------------------------------------------------
 
-% https://en.wikipedia.org/wiki/IPv6#IPv4-mapped_IPv6_addresses
--define(IPV4_IPV6_PREFIX, <<0:80, 16#FFFF:16>>).
+% "When storing IPv4 addresses in an IPv6 tree, they are stored as-is, so they
+%  occupy the first 32-bits of the address space (from 0 to 2**32 - 1)."
+%
+% * https://maxmind.github.io/MaxMind-DB/#ipv4-addresses-in-an-ipv6-tree
+%
+-define(IPV4_IPV6_PREFIX, <<0:96>>).
 
 %% ------------------------------------------------------------------
 %% API Record and Type Definitions