Use unreachable! instead of panic! in let-else to avoid LLVM coverage
artifact on closing brace. Remove cov-excl-start/stop from pipeline.rs.

Assisted-by: GLM-5.1 via pi

diff --git a/justfile b/justfile
index 9fb852a..645e289 100644
--- a/justfile
+++ b/justfile
@@ -46,6 +46,28 @@ coverage:
         exit 1
     fi
 
+coverage-html:
+    #!/usr/bin/env bash
+    set -euo pipefail
+    export RUSTFLAGS="-Cinstrument-coverage"
+    export CARGO_TARGET_DIR="target/coverage"
+    export LLVM_PROFILE_FILE="target/coverage/profraw/%p-%m.profraw"
+    rm -rf target/coverage
+    cargo test --workspace -q
+    rm -rf target/coverage/html
+    grcov target/coverage/profraw \
+        --binary-path ./target/coverage/debug/ \
+        -s . \
+        -t html \
+        --ignore-not-existing \
+        --keep-only 'src/**' \
+        --ignore 'src/bin/**' \
+        --excl-line 'cov-excl-line|unreachable!|tracing::' \
+        --excl-start 'cov-excl-start' \
+        --excl-stop 'cov-excl-stop' \
+        -o target/coverage/html
+    echo "HTML report at target/coverage/html/index.html"
+
 mutants:
     #!/usr/bin/env bash
     set -uo pipefail
diff --git a/src/ci/pipeline.rs b/src/ci/pipeline.rs
index e7b798b..6887f26 100644
--- a/src/ci/pipeline.rs
+++ b/src/ci/pipeline.rs
@@ -674,7 +674,7 @@ mod tests {
 (ci.job :orphan [:does-not-exist] (fn [_] nil))"#,
             "ci.fnl",
         );
-        let Err(e) = result else { panic!("expected validation error") }; // cov-excl-start
+        let Err(e) = result else { unreachable!() };
         let msg = e.to_string();
         assert!(
             msg.contains("CI validation failed"),
@@ -682,4 +682,3 @@ mod tests {
         );
     }
 }
-// cov-excl-stop