test(examples): avoid literal handle ids

Remove the last handwritten __h input from techdemo markup, switch E2E posts to generated handles, and extend example contracts to catch literal handle fields in templates.

req: dx/003

req: ceremony/002

req: examples/003

slhx-test/tests/examples_contract.rs

@@ -63,16 +63,25 @@ fn canonical_examples_do_not_author_low_level_resource_plumbing() {
         "::lower(include_str!",
         "lower_html(",
         "render_html(",
+        "name=\"__h\"",
+        "name='__h'",
     ];
     let mut failures = Vec::new();
     scan_examples(&examples, &mut |path, text| {
         if path.components().any(|part| part.as_os_str() == "tests") {
             return;
         }
-        if path.extension().and_then(|ext| ext.to_str()) != Some("rs") {
+        let Some(ext) = path.extension().and_then(|ext| ext.to_str()) else {
+            return;
+        };
+        if !matches!(ext, "rs" | "heml" | "html") {
             return;
         }
-        let runtime_source = text.split("#[cfg(test)]").next().unwrap_or(text);
+        let runtime_source = if ext == "rs" {
+            text.split("#[cfg(test)]").next().unwrap_or(text)
+        } else {
+            text
+        };
         for (line_no, line) in runtime_source.lines().enumerate() {
             if let Some(token) = forbidden.iter().find(|token| line.contains(*token)) {
                 failures.push(format!(