fix(bb): store pinned Chonk inputs under barretenberg as chonk-pinned-flows, downloaded once at build

[AztecBot]

Problem

bbapi/chonk_pinned_inputs.test.js fails consistently in the merge-train with ENOENT on a flow's ivc-inputs.msgpack mid-run (CI log d7647a841ee811a0): the native backend proves several flows, then the inputs directory disappears underneath it.

The pinned inputs were read from yarn-project/end-to-end/example-app-ivc-inputs-out — a working-tree directory under yarn-project/, shared across the parallel CI jobs and not owned by the barretenberg build. A concurrent yarn/e2e job cleaning that tree wipes the inputs out from under the bb test.

Fix

Make the pinned inputs a barretenberg build artifact, downloaded once, read everywhere, and give the directory a meaningful name:

• The canonical download/read directory is now barretenberg/cpp/chonk-pinned-flows (renamed from example-app-ivc-inputs-out), gitignored and owned by the barretenberg build — never touched by yarn-project/e2e jobs.
• The build already downloads it once via make bb-cpp-native-tests → bb-cpp-chonk-inputs → download_chonk_inputs; that now populates the new location, and the per-test chonk_inputs.sh download is an idempotent no-op against it.
• Every consumer reads the one canonical path: the bb.js and C++ test defaults, the IVC/browser/concurrency bench scripts, batch_verifier.bench.test.ts, plus docs/skills/launch.json.
• Live capture (refresh) is produced by the e2e stack, so it uses yarn-project/end-to-end/chonk-pinned-flows via a separate chonk_capture_dir(); chonk_inputs.sh update uploads from there. Read path and capture path are distinct.

download_pinned_chonk_inputs is a plain extract (no symlink/store layer).

Testing

No native bb build here, so not the prove loop itself, but the path/wiring is exercised:

• chonk_inputs.sh download creates a real barretenberg/cpp/chonk-pinned-flows with all flows; it is gitignored.
• pinned_chonk_inputs_dir → barretenberg/cpp/chonk-pinned-flows, chonk_capture_dir → e2e path.
• All shell scripts parse; repo-wide scan confirms no remaining example-app-ivc-inputs-out references.