ci(clippy): persistent cache + pre-baked registry for fast warm builds

Two root causes for clippy taking 100+ minutes on cold runners:
1. CARGO_HOME=$CI_PROJECT_DIR/.cargo (global default) is wiped each job,
   forcing re-download of the entire crate registry on every run.
   Fix: override CARGO_HOME=/root/.cargo (pre-baked in the CI image).
2. CARGO_TARGET_DIR not persisted — compiled artifacts discarded after
   each run, forcing full recompile on the same runner.
   Fix: same /cache/cargo-target/$CACHE_KEY/clippy setup as test jobs.

First run on a fresh runner: still slow (~100m, cold compile).
Subsequent runs on same runner: ~10-15m (warm cache reuse).
Timeout bumped to 3h to safely cover the cold-build case.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.gitlab-ci.yml

@@ -169,7 +169,20 @@ clippy:
   needs: [fmt]
   variables:
     SKIP_WASM_BUILD: "1"
-  timeout: 2h
+    # Use pre-baked registry in the CI image instead of re-downloading all
+    # crates on every run (global CARGO_HOME=$CI_PROJECT_DIR/.cargo is wiped).
+    CARGO_HOME: /root/.cargo
+  timeout: 3h
+  before_script:
+    - rustup show
+    - rustc --version && cargo --version
+    - rustup target add wasm32v1-none
+    - export CACHE_KEY="$(cat Cargo.lock rust-toolchain.toml | md5sum | cut -c1-12)"
+    - export CARGO_TARGET_DIR="/cache/cargo-target/${CACHE_KEY}/clippy"
+    - mkdir -p "$CARGO_TARGET_DIR"
+    - find /cache/cargo-target -mindepth 1 -maxdepth 1 -type d ! -name "${CACHE_KEY}" -exec rm -rf {} + 2>/dev/null || true
+    - echo "CARGO_TARGET_DIR=$CARGO_TARGET_DIR (disk $(df -h /cache/cargo-target | tail -1 | awk '{print $3 "/" $2 " used, " $4 " free"}'))"
+  cache: []
   script:
     - git config --global --add safe.directory '*'
     - |