Create: Vertical Chain Conveyors

## What's changed - Mixed-axis chain links are no longer impossible on coplanar layouts. A wall-mounted conveyor can now chain diagonally down to a floor conveyor in front of it (or up to a ceiling conveyor) without enabling `allowSteepMixedAxisConnections`. - Because perpendicular wheels force the chain through a 90-degree change of plane, mixed-axis links now let the chain leave each wheel up to 67.5 degrees out of that wheel's plane. In practice, wall-to-floor diagonals connect at any 1:1 scale and at slopes between roughly 1:2.4 and 2.4:1. - Same-axis links (floor to floor, floor to ceiling, wall to the same wall) keep Create's exact 45-degree rule. - Wheel-clearance, distance, chain-cost, and connection-count checks are unchanged. `allowSteepMixedAxisConnections` still permits the remaining very lopsided mixed-axis links.

## What's changed - Updated the mod logo with a Create-style blueprint circle background. - Added a centered logo to the top of the README. - Added a high-quality Modrinth project icon variant under the 256 KiB upload limit. - Included the updated in-game mod-list icon and Create asset attribution.

## What's changed - Added the synchronized server setting connections.allowSteepMixedAxisConnections, disabled by default. - Enabling it permits steep mixed-axis links such as coplanar floor-to-wall corners while preserving same-axis slope limits. - Retained wheel-clearance, distance, chain-cost, and connection-count checks. - Enforced the setting in client placement previews and on the server connection packet path. - Added focused policy tests and configuration documentation. Related: #3

See the corresponding GitHub release for details.