[codex] Add GCP machine guidance for Fusion Snapshots

[llewellyn-sl]

Summary

This draft adds concrete machine-selection guidance for Google Cloud Batch Fusion Snapshots.

Source context

• Slack source: https://seqera.slack.com/archives/C0APYR2A3A8/p1775081447163959
• Jira: https://seqera.atlassian.net/browse/EDU-1113

Doc target

• fusion_docs/guide/snapshots/gcp.md

Rationale

The EDU ticket pointed to a documentation gap around which machine types are suitable for Fusion Snapshots on Google Batch. The existing docs already described the feature, but they did not connect snapshot reliability to local SSD support or give a practical starting point for production sizing.

Validation

• Reviewed the edited page directly
• Confirmed the new guidance aligns with the existing Google Batch Fusion guidance already documented elsewhere in the docs repo
• Did not run a full docs build in the temp automation clone

Follow-up

Please sanity-check whether we want additional cross-links from compute environment docs back into the Fusion Snapshots guide.

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[llewellyn-sl]

Superseded by #1273 because this draft was opened against the wrong base branch and picked up unrelated commits. Please use #1273 for review.