feat: show a block's density in its item tooltip

[adamico]

An item a Density tag names shows one grey line right under its name, its Density and the motes one yields fed to a Void Displacer:

Density 4: 64 motes

• The motes come from the server's densityMotes, so a pack that changes the curve changes the line. Before a world has loaded the config, the line is the Density alone.
• Other mods' storage blocks in the c: tags show it too; an item of Density 0 shows nothing.
• The Siphon's yield is left out: it is one Density step below a Displacer's, never below 1, and naming both wrapped the tooltip.
• One mote is said as one: "Density 1: 1 mote".
• A tooltip only, no screen and no Jade line: a placed block's Density never matters, only a fed one's.

Game tests build real tooltips on the server: diamond block 4 and 64, copper block 1 and 1, cobblestone no line. Reviewed with /code-review (Standards and Spec), its small findings applied.

Closes #106

🤖 Generated with Claude Code