ci: constrain benchmark cache access

[philip-gai]

Summary

• Cache producers: enforce write-only for jobs that seed fresh Maven/JDK entries or benchmark uploads to unique keys.
• Cache consumers: enforce read for measurement jobs that restore seeded entries without mutating them.
• Key computation: enforce none for the probe job, which inspects deterministic cache keys without reading or writing cache data.

Why job-scoped

• The workflows deliberately separate cache producers from consumers, so job-level modes express each boundary without changing preparation, reporting, or artifact-only jobs.
• Existing action inputs such as cache-read-only remain intact. Platform cache-mode adds an independent enforcement layer around the job.

Validation

• npm test (66 tests)
• Parsed all changed workflow YAML files with Ruby Psych
• git diff --check

[Copilot]

Pull request overview

This PR tightens GitHub Actions cache access in benchmark workflows by adding job-scoped cache enforcement, separating cache producers (seed/upload) from consumers (measurement), and disabling cache access entirely for key-probe jobs.

Changes:

• Add job-level cache-mode to seed/upload jobs to constrain cache writes.
• Add job-level cache-mode: read to measurement jobs to prevent cache mutation.
• Add job-level cache-mode: none to the cache-key stability probe job to avoid cache access while computing deterministic keys.

Show a summary per file

File	Description
.github/workflows/jdk-cache.yml	Adds cache access mode boundaries between seed (producer) and measure (consumer) jobs.
.github/workflows/cache-value.yml	Marks seed as cache producer and measure as read-only cache consumer via job-level cache mode.
.github/workflows/cache-save.yml	Constrains the cache-save measurement job’s cache access mode for uploads.
.github/workflows/cache-key-stability.yml	Disables cache access for the probe job that inspects deterministic cache keys.

Review details

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• Files reviewed: 4/4 changed files
• Comments generated: 3
• Review effort level: Lite