feat: add Minecraft Forge 26.3 scroll

[druid-infra]

Summary

• Add the missing Minecraft Forge 26.3 scroll.
• Follow the existing Forge 26.2 scroll layout and runtime behavior.
• Update the app version and Forge snapshot-cache artifact reference to 26.3.

Live version checks

• Minecraft Vanilla latest release: 26.3 (Mojang version manifest)
• PaperMC latest stable version group: 26.3 (Paper downloads API)
• Spigot latest published version metadata: 26.3
• Forge latest release: 26.3-66.0.0 (Forge Maven metadata)

Vanilla, PaperMC, and Spigot 26.3 scrolls are already present on master; Forge 26.3 was missing.

Validation

• bash -n passed for all new Forge shell scripts.
• Forge 26.3 installer URL returned HTTP 200.
• Spigot 26.3 metadata returned HTTP 200.
• Go tests could not run because Go is not installed in the execution environment.

[druid-infra]

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