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

Nobody has claimed this yet.

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entries that contradict their own repo's `pyproject.toml` at the same commit (`uv lock --check` reports the lockfile out of date).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`.

Start by reading DESIGN-worker-registry-core-split.md Phase 4 and the attractor HISTORY-MAP.md, then inspect the DirectProviderBackend shim and its eligibility checks. Re-run the stated repository and lockfile checks against the listed main branches; done means recording any outstanding condition and whether the shim can be retired.

Written by the indexing model from the issue text.