ci: make Cargo with nextest the CI Gate Rust lane and retire the Bazel jobs (#1618)

[DecisionNerd]

Sub-issue of #1618 (ADR 0048). It implements decision items 1–3 and 5 for the PR gate.

Problem

The CI Gate Rust lane is the bazel-bootstrap job (//:ci_rust_tests, --config=correctness). ADR 0048 moves that lane to Cargo with nextest.

Scope

1. Replace bazel-bootstrap in .github/workflows/test.yml with a Cargo Rust-tests job on blacksmith-4vcpu-ubuntu-2404:
   • a sticky target/ volume;
   • cargo nextest run --workspace --locked;
   • the two custom-harness targets and the doctests under cargo test --workspace --locked;
   • dev/test profile, so debug assertions and overflow checks stay on.
2. Keep the non-Bazel steps the job runs today, with Cargo-built binaries where they used Bazel ones:
   • the offline progressive provider tests;
   • the tiny and ownership-growth lifecycle producer, using a Cargo-built gf;
   • the binding packaging smoke.
3. Retire bazel-diagnostics and the Bazel-only steps: drift check, migration ledger, suite-membership inventory, and cache-perf policy.
4. Gate the new job on Rust inputs through scripts/ci/classify-changes.sh. Rename or retire the bazel output, and update scripts/ci/test-classify-changes.sh.
5. Update the ci-gate needs, config/gate-registry.json, and scripts/ci/test-ci-storage-policy.py expectations. CI Gate stays the sole required status.

Non-goals

• Deleting BUILD.bazel and the rest of the Bazel tooling (sub-issue 3).
• Binding RC (sub-issue 2).

Acceptance criteria

• On a Rust-changing PR, CI Gate runs the Cargo lane and no Bazel job. The run log shows the nextest summary (≥ 5,100 tests) plus the custom-harness targets and the doctests.
• A docs-only PR skips the Rust lane and CI Gate passes.
• The lifecycle producer runs against a Cargo-built gf.
• make gate-registry-check passes, and CI Gate remains the sole required status.