Keep old Base Metals items when upgrading 1.12 worlds

[SkyBlade1978]

This fixes older Base Metals worlds losing Mercury blocks, carbon powder, iron nuggets and metal door items when upgraded to 1.12.2.

The missing-mapping handlers now run correctly and cover all 20 historical item names plus the old Mercury block name. A data fixer rewrites old stack names before Forge reads their contents, keeping counts, damage, enchantments, names and other NBT.

It also fixes the less obvious case where a chunk is first visited after the upgraded world has already been saved and restarted. The new CI fixture leaves a distant chest unopened across restarts, checks that its region stays unchanged, then visits it and checks another reload. Disabling stack recovery in a throwaway jar reproduced the item loss at that later visit.

Checks completed locally:

• 52 unit tests, clean build and Javadocs.
• The full packaged-server matrix, including content modes, Tinkers/Armory, TAIGA, Ancient Warfare, CoFH and OreSpawn migration/rejection checks.
• First load, reload, delayed-chunk migration and a final stability reload.
• Release-JAR audit, checksums and Eclipse classpath isolation.

This prepares 2.6.1.112021. No registry entries or gameplay rules are changed. The README and changelog include the upgrade instructions and explain that other mods still need their own compatible versions. Agent notes, private worlds and generated test output are not included.