fix: support Paper 1.19.4 and skip degenerate render lines

[twme-ai]

Summary

• lower api-version to 1.19 so Paper 1.19.4 can load the plugin
• compile WorldEditDisplay to Java 17 bytecode and document server-specific Java requirements
• skip non-finite and sub-millimeter line segments before TextDisplayShapes validation
• add regression coverage for the plugin descriptor and line validation boundary

Context

The 2.4.0 compatibility matrix found three failures:

1. Paper 1.19.4 rejected the plugin because api-version was 1.20.
2. Paper 1.19.4/1.20 legacy remappers could not inspect Java 21 class files. The shaded dependency was fixed and published by fix: target Java 17 bytecode TextDisplayShapes#28.
3. Cylinder tangent grids and ellipsoid pole rings generated zero-length lines, causing IllegalArgumentException: line endpoints must be distinct and partial renders.

The common renderLine boundary now uses the same squared-length threshold (1e-6) as TextDisplayShapes, which also protects other renderers from invalid generated geometry.

Verification

• mvn clean package: 34 tests passed
• clean Maven repository build against published TextDisplayShapes snapshot build 6
• verified every class in the shaded JAR is class file major 61 or lower
• exercised five CUI region types on the latest stable Paper build for all 21 released versions from 1.19.4 through 26.2
• all 21 versions produced positive renderer/retained-line counts and client-side text_display spawn packets
• no WED renderer, API-version, invalid-plugin, or legacy-remapper errors in the final logs

Paper versions tested: 1.19.4; 1.20, 1.20.1, 1.20.2, 1.20.4, 1.20.5, 1.20.6; 1.21, 1.21.1, 1.21.3, 1.21.4, 1.21.5, 1.21.6, 1.21.7, 1.21.8, 1.21.9, 1.21.10, 1.21.11; 26.1.1, 26.1.2, 26.2. The 26.2 client packet check used a 26.1.2 Mineflayer client through ViaVersion/ViaBackwards because minecraft-data does not yet ship a 26.2 registry; the Paper 26.2 server and WED execution were native.