Guard the sharpening kernel against WGSL reserved keywords

The fused-fragment check in lib.rs cannot see a detail pass: it is a
separate shader composed at a resolution compose() never knows. The
kernel's own test now scans the block the composer wrapped, with comments
stripped so prose about the keyword cannot fail a test about the code.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

core/dr-pipeline/src/ops/capture_sharpen.rs

@@ -799,9 +799,8 @@ mod tests {
         // source rather than at the operation that wrote it, and this body
         // reaches for exactly that word.
         //
-        // Not the full reserved list; the words a convolution would plausibly
-        // pick for a local.
-        // The same list `no_fragment_declares_a_wgsl_reserved_keyword` uses,
+        // Not the full reserved list; the words a kernel would plausibly pick
+        // for a local. The same list `no_fragment_declares_a_wgsl_reserved_keyword` uses,
         // kept identical on purpose: two lists that drift apart would let a
         // word be safe in one stage and not in the other, which is the sort of
         // difference nobody discovers until a shader fails to compile.
@@ -813,18 +812,33 @@ mod tests {
         ];
 
         for pass in chain_at(&sharpening(100.0, 2.0), RenderScale::full((512, 512))).passes {
-            // Only what this operation wrote. The composer's own preamble
-            // declares `var output` and `let coord`, which naga accepts and
-            // which are not this test's business.
-            let body = pass
+            // Only what this operation wrote — the block the composer wraps
+            // the body in. Its preamble declares `var output` and its tail
+            // writes through it, and neither is this test's business; scanning
+            // the whole file would fail on generated code nobody here can fix.
+            let block = pass
                 .source
                 .rsplit_once("    {\n")
-                .expect("the operation's block")
+                .expect("the operation's block opens")
                 .1;
+            let body = block
+                .split_once("\n    }\n")
+                .map_or(block, |(inside, _)| inside);
+
+            // Comments are not declarations, and the kernel deliberately names
+            // `target` in one to say why it does not use it. Stripping them
+            // keeps this on the code, so that editing the prose can never fail
+            // a test about what compiles.
+            let code: Vec<&str> = body
+                .lines()
+                .filter(|l| !l.trim_start().starts_with("//"))
+                .collect();
+            let code = code.join("\n");
+
             for keyword in RESERVED {
                 for form in [format!("let {keyword} "), format!("var {keyword} ")] {
                     assert!(
-                        !body.contains(&form),
+                        !code.contains(&form),
                         "{} declares `{keyword}`, which is a WGSL reserved keyword",
                         pass.label
                     );