ci: make clippy a hard gate

The advisory step existed because the tree carried a warning backlog. Measured
on 1.97.1 — the pinned toolchain CI actually uses — that backlog is three
warnings, not the ~51 the comment claimed: two unnecessary_sort_by in
smart_cache and one redundant into_iter in offline. Fixed, so clippy now runs
with -D warnings and a warning means new breakage.

Worth recording why this took a toolchain pin to do safely: the same tree
measured 0 warnings on 1.92.0 and 3 on 1.97.1. Flipping the flag on a local
measurement, without the pin, would have reddened CI on the next push.

TRACES: | DR-206

.gitea/workflows/build-and-test.yml

@@ -84,21 +84,18 @@ jobs:
           cd src-tauri
           cargo fmt --all -- --check
 
-      # ⚠️ Advisory for now — clippy warnings do NOT fail this job yet.
+      # Clippy is a hard gate. It was advisory while the tree carried a warning
+      # backlog; that backlog is gone (0 warnings on 1.97.1, the pinned
+      # toolchain), so a warning here is now new breakage rather than old noise.
       #
-      # The tree carries ~51 pre-existing warnings; adding `-D warnings` today
-      # would paint CI red on unrelated work. A compile *error* still fails the
-      # step, so this is not a no-op: it stops new breakage and surfaces the
-      # backlog in every run.
-      #
-      # TODO: once the existing warnings are cleared, tighten this to
-      #   cargo clippy --all-targets -- -D warnings
-      # Flip that flag — do not delete the step. Track progress with
-      # `cd src-tauri && cargo clippy --all-targets 2>&1 | grep -c '^warning'`.
-      - name: Run clippy (advisory)
+      # This only means anything because src-tauri/rust-toolchain.toml pins the
+      # compiler: clippy's lint set moves between releases, so an unpinned gate
+      # would fail on whatever the runner happened to install. The pin and this
+      # flag stand or fall together — if you unpin, drop this back to advisory.
+      - name: Run clippy
         run: |
           cd src-tauri
-          cargo clippy --all-targets
+          cargo clippy --all-targets -- -D warnings
 
       - name: Run Rust tests
         run: |