feat: add Minecraft Forge 26.3 scroll

[druid-infra]

Summary

• Add the missing Minecraft Forge 26.3 scroll.
• Copy the existing Forge 26.2 layout and update version-specific references to 26.3.

Upstream checks

• Mojang version manifest latest release: 26.3.
• Paper API latest project version: 26.3.
• Spigot versions index includes 26.3.
• Forge Maven metadata includes Forge artifacts for Minecraft 26.3 (latest listed 26.3-66.0.9).
• Existing repository coverage already included Vanilla, PaperMC, and Spigot 26.3, but Forge stopped at 26.2.

Validation

• Parsed the new scroll.yaml successfully and asserted app_version: "26.3".
• Confirmed the new Forge tree contains no stale 26.2 references.
• Repository validator could not run because the environment does not have Go installed (go: command not found).

[druid-infra]

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