block: generalize secondary break drops

[HashimTheArab]

Summary

• move contained and special break drops into block.BreakInfo.AdditionalDrops
• expose the complete drop list to HandleBlockBreak before block/inventory mutation, with pure pre-event evaluation and cancellation safety
• migrate container, campfire, chest, composter, decorated pot, item frame, jukebox, and shulker behavior; preserve double-chest half inventory semantics
• apply the generalized drops to explosions while keeping ordinary drops chance-gated and shulker boxes intact

Verification

• go test ./...
• go vet ./...
• go test -race ./server/block ./server/player
• git diff --check

No tests or test-file changes were added, per request.