[SkyBlade1978]

I've added diamond deposits to the 1.10.2 branch and improved the abandoned mine entrances.

• Rare diamond pipes have patchy ore, wider upper sections and narrow roots. Nearby river pockets can contain diamond-bearing gravel.
• Rich gravel keeps the 1-in-16 diamond chance, normal Fortune rewards and Silk Touch behaviour. Spent ordinary gravel cannot produce more diamonds.
• New iron, coal and diamond workings have stepped entrances, turning landings, timber supports and guardrails instead of long ladder shafts. Attempts are enabled by default where the site is safe; river placers are left alone.
• Existing catalogues, world settings, terrain and saved mine layouts are preserved, including old mines with sections that have not been explored yet.
• The README, changelog and guides now cover the new deposits, settings and testing profiles.

The automated checks pass: 89 unit tests, 18 helper tests, 415 packaged-server boots, both full policy/import matrices, reloads, later chunks and older-world upgrades. Two clean builds produce identical jars. The new entrances have also been tested in game.

This prepares 0.3.0.110021 as a beta, using the published OreSpawn 4.1.4.110021 dependency.