DirectProviderBackend deprecation-shim: eligibility check now that attractor's compat window is closed

[bkrabach]

Filed from microsoft/amplifier-bundle-attractor PR #346 (the P4 slim, attractor-28x), which deleted this repo's twelve compat-window module copies from attractor. That deletion was the stated precondition for reviewing the DirectProviderBackend deprecation shim here.

Ask: re-run the shim's own eligibility criteria now that no consumer resolves the engine from amplifier-bundle-attractor any more. The pre-flight gate on that PR verified, from fresh --depth 1 clones of amplifier-app-actions, amplifier-resolve, amplifier-resolver-dot-graph, and amplifier-app-wiki-weaver at their main tips, that zero live install references to attractor modules remain; the only two uv.lock mentions left are stale transitive entries that contradict their own repo's pyproject.toml at the same commit (uv lock --check reports the lockfile out of date).

If the shim's retirement condition was 'once the old source is gone', it has fired. If it was something narrower, this issue is the place to say so and record what is still outstanding.

Context: DESIGN-worker-registry-core-split.md Phase 4; attractor HISTORY-MAP.md.