ci: skip cross-target check for Cargo.lock removals (#8866)

A Cargo.lock change no longer forces the cross-target cargo check by
itself. The check runs when the lock diff adds lines, or when a removed
package is still referenced from Rust sources.

Assisted-by: Claude Code:claude-opus-5-5

.github/workflows/ci.yaml

@@ -109,7 +109,6 @@ jobs:
           fi
 
           if ! git diff --quiet "${MERGE_BASE}...HEAD" -- \
-            ':Cargo.lock' \
             ':rust-toolchain.toml' \
             ':.cargo/**' \
             ':.github/workflows/ci.yaml' \
@@ -163,6 +162,21 @@ jobs:
             exit 0
           fi
 
+          # Added or bumped packages bring new code to every target. Removed
+          # packages only matter while cfg-gated code still references them.
+          lock_diff=$(git diff -U0 "${MERGE_BASE}...HEAD" -- Cargo.lock)
+          if grep -Eq '^\+[^+]' <<< "${lock_diff}"; then
+            echo "changed=true" >> "$GITHUB_OUTPUT"
+            exit 0
+          fi
+          while IFS= read -r crate; do
+            crate=${crate//-/_}
+            if git grep -qE "(^|[^A-Za-z0-9_])${crate}::|(use|crate) ${crate}([^A-Za-z0-9_]|$)" -- '*.rs'; then
+              echo "changed=true" >> "$GITHUB_OUTPUT"
+              exit 0
+            fi
+          done < <(sed -n 's/^-name = "\(.*\)"$/\1/p' <<< "${lock_diff}")
+
           # Platform cfg, or FFI crates that only exist on some targets.
           platform_re='target_os|target_arch|target_family|target_env|target_vendor|target_pointer_width|target_endian|cfg\(windows\)|cfg\(unix\)|cfg\(wasi|not\(windows|not\(unix|\[target\.|\blibc::|\bnix::|windows_sys|winapi::|rustix::|std::os::'