ci(hailo): point cache keys at the workspace-root Cargo.lock (iter 227)

iter-219 (workspace re-inclusion, ADR-178 Gap E) removed the
per-crate `crates/ruvector-hailo-cluster/Cargo.lock` — but the
hailo-backend-audit workflow's two `actions/cache@v4` keys still
hashed that now-missing path:

    key: ${{ runner.os }}-cargo-${{ hashFiles('crates/ruvector-hailo-cluster/Cargo.lock') }}

`hashFiles()` returns an empty string when the pattern matches
nothing. So both cache keys would have collapsed to the constant
prefix `${{ runner.os }}-cargo-` (and `-cargo-test-`) on every run —
every PR, every branch, every commit would have shared the same
cache slot, defeating the cache invalidation iter-178 set up.
Either falsely-stale build artifacts on a dep change, or chronic
cache misses depending on how the runners' eviction policy
shook out.

Fix: point both keys at the workspace-root `Cargo.lock`, which is
canonical post-iter-219. Same parallel as iter-224's cargo-audit
fix that handled the matching deny-vs-audit drift.

Validated:
  - yaml parses (`python3 -c 'import yaml; yaml.safe_load(...)'`)
  - root Cargo.lock exists at the new path
  - Pattern matches GitHub Actions' relative-to-GITHUB_WORKSPACE
    semantic for `hashFiles()` — Cargo.lock at repo root is
    correctly resolved without a path prefix.

Pure CI hygiene; no code change. Catches the third post-iter-219
side effect (after iter-224's cargo-audit ignores and iter-226's
real-hardware verification).

Co-Authored-By: claude-flow <ruv@ruv.net>

.github/workflows/hailo-backend-audit.yml

@@ -97,7 +97,13 @@ jobs:
             ~/.cargo/registry
             ~/.cargo/git
             crates/ruvector-hailo-cluster/target
-          key: ${{ runner.os }}-cargo-${{ hashFiles('crates/ruvector-hailo-cluster/Cargo.lock') }}
+          # Iter 227 — cache key now points at the workspace-root
+          # Cargo.lock since iter-219 removed the per-crate
+          # `crates/ruvector-hailo-cluster/Cargo.lock`. Keeping the
+          # stale reference would have resolved to an empty hash via
+          # `hashFiles()` of a missing path, collapsing every branch's
+          # cache to the same key → false hits on unrelated trees.
+          key: ${{ runner.os }}-cargo-${{ hashFiles('Cargo.lock') }}
       - name: Run clippy
         working-directory: crates/ruvector-hailo-cluster
         run: cargo clippy --all-targets -- -D warnings
@@ -116,7 +122,8 @@ jobs:
             ~/.cargo/registry
             ~/.cargo/git
             crates/ruvector-hailo-cluster/target
-          key: ${{ runner.os }}-cargo-test-${{ hashFiles('crates/ruvector-hailo-cluster/Cargo.lock') }}
+          # Iter 227 — workspace-root Cargo.lock (see clippy job).
+          key: ${{ runner.os }}-cargo-test-${{ hashFiles('Cargo.lock') }}
       - name: Run all suites
         working-directory: crates/ruvector-hailo-cluster
         run: cargo test