Consolidate near-copy tune functions

[EmilHvitfeldt]

Several functions in tidyclust are near-copies of functions in tune, differing only in small cluster-specific ways. We should evaluate each one and either: import the tune version directly if the difference can be handled cleanly, or move the shared logic into tune and keep only the cluster-specific part in tidyclust.

All changes should be verified carefully — the differences are subtle and intentional in some cases.

Functions to evaluate

Function(s) in tidyclust	Match in tune	Exported from tune?	Key difference
merge.cluster_spec() / update_model() / update_recipe()	merge.model_spec() etc.	❌ No	Checks source == "cluster_spec" vs "model_spec"
check_workflow()	check_workflow()	❌ No	Has cluster-specific guards (check_installs on cluster spec)
check_metrics()	check_metrics()	❌ No	Checks for cluster_metric_set instead of generic metric sets
load_pkgs.cluster_spec()	load_pkgs.model_spec()	✅ Yes (generic)	Dispatch on cluster_spec; infra_pkgs list differs slightly

Notes

• merge.cluster_spec() / update_model() / update_recipe(): the source == "cluster_spec" check is a real distinction. One option is to make tune's version accept a source argument; another is to keep the tidyclust copies but reduce them to only the differing line.
• check_workflow() and check_metrics() need to stay in some form but the bulk of their logic could live in tune with a hook for cluster-specific validation.
• load_pkgs.cluster_spec() is likely fine to keep as-is since it correctly dispatches on the cluster-specific class — verify the infra_pkgs difference is intentional.