Fix grapevine stem z-fighting

[Alatyami]

Summary

• prevent center and vertical linen rope models from rendering inside mature grapevine trunks
• prevent the center rope from overlapping connected grapevine leaf trunks
• remove the redundant leaf-trunk bottom cap and slightly inset the mature vine top face
• apply the blockstate corrections consistently to purple, red, and white grapevines

Root cause

Grapevine blockstates layered linen rope geometry over wooden stem geometry. Coplanar faces were visible as z-fighting when a player stood inside the non-colliding vine blocks and looked along the stem.

Validation

• ./gradlew.bat test -x createMinecraftArtifacts
• git diff --check
• manually verified in game while looking both up and down from inside connected grapevine and leaf blocks

Closes #132