feat: add Minecraft 26.3 support

[darbyjack]

Summary

• Update Spigot API to 26.3-R0.1-SNAPSHOT and XSeries to 14.0.0; retain the existing shaded dependencies and relocations.
• Add the Java 25 runServer26_3 target and make runServer use Minecraft 26.3. All existing historical targets remain.
• Update potion integration to XSeries 14's XPotion.of and XTag.DEBUFFS APIs.
• Recover from XSeries 14 profile reflection failures so GUI skulls fall back to a legacy-compatible player head instead of throwing.

Validation

• ./gradlew check legacyApiProbeClasses shadowJar --rerun-tasks --no-daemon --stacktrace — passed.
• ./gradlew testJava11 --rerun-tasks --no-daemon — passed, 68 tests, 0 failures, on a Java 11 test executor.
• Legacy 1.8.8 API probe — passed; 2 direct Material references checked, 0 allowlisted.
• Shaded JAR built; XSeries classes remain relocated under me.glaremasters.guilds.libs.xseries.
• Paper 26.3 build 177 on Java 25 loaded Guilds to Ready to go!; the guilds command returned its help listing.
• Plugwright E2E on 1.21.8 — 136 passed, 0 failed, 0 skipped.
• Secret scan — passed.

Limitations

• Plugwright's current protocol data (minecraft-data 3.117.0) recognizes Minecraft 26.1 but not 26.3, so E2E remains on 1.21.8.
• When XSeries 14's pre-26.3 profile reflection fails, custom skull textures fall back to plain player heads so Guilds GUIs continue working.
• Quark reports one nonfatal dependency-resolution error (18 of 19 resolved dependencies loaded) on the test servers; Guilds still reaches Ready to go!.