Crossplane client GVK caches in single-version clients are dead code

[jcogilvie]

Summary

Three of the Crossplane clients populate a `c.gvks` cache via `GetGVKsForGroupKind` during `Initialize` but never read it — the actual list/get paths use a hardcoded single-version GVK. The cache is dead storage today; if Crossplane ever ships a v2 of any of these resources, the hardcoded paths will silently miss the new versions.

Affected clients

File	Cached group	Hardcoded list GVK	TODO present?
`cmd/diff/client/crossplane/composition_client.go`	`apiextensions.crossplane.io`	`apiextensions.crossplane.io/v1, Composition`	yes
`cmd/diff/client/crossplane/composition_revision_client.go`	`apiextensions.crossplane.io`	`apiextensions.crossplane.io/v1, CompositionRevision`	no (same pattern)
`cmd/diff/client/crossplane/function_client.go`	`pkg.crossplane.io` (was wrong group until #309)	`pkg.crossplane.io/v1, Function`	yes

For comparison, `definition_client.go` (XRDs) and `environment_client.go` (EnvironmentConfigs) use the same pattern correctly: they call `listMatchingResources(ctx, c.client, c.gvks, ...)` to fan out across every discovered version.

Background

The cache and `GetGVKsForGroupKind` were introduced in commit c155e30 to support the v1↔v2 transition for XRDs and EnvironmentConfigs. The pattern was retrofitted onto Composition/CompositionRevision/Function clients but the read side was never wired up. The TODOs at `composition_client.go:103` and `function_client.go:80` capture the known reason: those clients use typed structs from `apiextensionsv1`, so iterating versions requires per-version unstructured→typed conversion code that doesn't exist yet.

Consequences

• Today: zero functional impact — Crossplane only ships v1 of Composition, CompositionRevision, and Function, so the hardcoded GVK matches reality.
• If Crossplane ships v2 of any of these: silent diffs against the wrong/missing version. Lookups would either return nothing or fail to decode v2 unstructured into the v1 typed struct.
• Function client until chore(deps): update dependency golangci/golangci-lint to v2.12.2 #309: the discovery query used the wrong group (`apiextensions` instead of `pkg`), so `c.gvks` was always an empty slice. PR chore(deps): update dependency golangci/golangci-lint to v2.12.2 #309 fixed the group, but the field is still unread.

Resolution options

Pick one:

1. Delete the dead cache. Remove the `gvks` field, the `GetGVKsForGroupKind` call in `Initialize`, and any related plumbing from the three clients above. Cheapest fix; defers multi-version support to whenever Crossplane actually ships it.
2. Wire up multi-version listing. Use `c.gvks` + `listMatchingResources` in those clients, and add per-version conversion to the typed structs. Closes the future gap proactively. More work.

Option 1 reflects current reality; option 2 is what the TODOs are gesturing at. Either is fine — picking deliberately is the point.