Retry transient Forge cold-bootstrap downloads

[SkyBlade1978]

Summary

• retry the deliberately cold Forge/Mavenizer bootstrap up to three times
• discard partial Forge and project Mavenizer state before each retry
• preserve the downloaded Gradle/tool dependencies and the successful cache for the existing idempotence check
• retain a hard failure after the third unsuccessful attempt
• add a workflow contract test for the bounded retry and cleanup behavior

Why

The post-merge 1.21.1 CI run completed the full build, all tests and release artifact audits successfully. Only the empty-cache bootstrap failed when the Forge Mavenizer's mapping download received a transient java.net.SocketException: Connection reset during mergeMappings.

Failed run: https://github.com/MinecraftModDevelopmentMods/OreSpawn/actions/runs/33540896454

Validation

• actionlint 1.7.12 passes for ci.yml
• focused ReleaseWorkflowContractTest passes under the pinned Java 21/8/25 toolchains
• retry preserves the exact Gradle argument vector and log output
• partial Forge/Mavenizer state is removed before retrying
• persistent failures remain fatal after three attempts
• no production source, gameplay, API, schema or release artifact changed

This PR does not create a tag or publish a release.