examples: bpfman pre-load gate adapter

[ErenAri]

Companion to a proposed hook in bpfman: ErenAri/bpfman@preload-validation-gate.

Design

bpfman defines a tiny interface; we supply the implementation. bpfman takes no dependency on bpfcompat — that was the constraint worth designing around, since a hard dep would be an unreasonable ask of another project.

// in bpfman
type PreLoadValidator interface {
    ValidateBeforeLoad(ctx context.Context, objectPath string) error
}

// in the consumer's wiring
mgr = mgr.WithPreLoadValidator(bpfmangate.New())

Why the interface is shaped this way

It mirrors bpfman's existing ProgramValidator convention rather than inventing a new one. The two ask different questions: ProgramValidator is structural (do these program names exist in this object), this one is about the kernel (will the verifier accept it here). An object can name every program correctly and still be rejected for a missing helper, map type, or BTF relocation target.

The error is the product. bpfman surfaces it verbatim, so a refusal carries the classification code, the verifier's own reason, and the errno — instead of the bare EINVAL the loader produces today.

Verified

• Builds with and without (the no-tag stub reports the feature is unavailable rather than silently passing).
• On the bpfman side: their full manager suite passes with the patch, plus two new tests — a refusal blocks the load and leaves clean state, and the default nil validator is a no-op. (platform/image/oci fails there, but identically with my changes stashed — pre-existing.)

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