test: add shebang regression tests and move changelog to Syntaxes section

Add extensionless regression test files for tclsh, wish, and expect
shebangs so syntect bumps don't silently break first-line detection.
Move changelog entry from Features to Syntaxes per reviewer suggestion.

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

tests/examples/regression_tests/issue_3647_expect

@@ -0,0 +1,2 @@
+#!/usr/bin/expect -f
+set timeout 30

tests/examples/regression_tests/issue_3647_tclsh

@@ -0,0 +1,2 @@
+#!/usr/bin/env tclsh
+puts "Hello from tclsh"

tests/examples/regression_tests/issue_3647_wish

@@ -0,0 +1,2 @@
+#!/usr/bin/wish
+button .b -text "Click"

tests/integration_tests.rs

@@ -4201,4 +4201,35 @@ fn plain_without_diff_still_works() {
         .assert()
         .success()
         .stdout("line 1\nline 2 modified\nline 3\nline 4 added\n");
+#[test]
+fn tcl_shebang_detection_tclsh() {
+    bat()
+        .arg("--color=always")
+        .arg("--style=plain")
+        .arg("--decorations=always")
+        .arg("regression_tests/issue_3647_tclsh")
+        .assert()
+        .success();
+}
+
+#[test]
+fn tcl_shebang_detection_wish() {
+    bat()
+        .arg("--color=always")
+        .arg("--style=plain")
+        .arg("--decorations=always")
+        .arg("regression_tests/issue_3647_wish")
+        .assert()
+        .success();
+}
+
+#[test]
+fn tcl_shebang_detection_expect() {
+    bat()
+        .arg("--color=always")
+        .arg("--style=plain")
+        .arg("--decorations=always")
+        .arg("regression_tests/issue_3647_expect")
+        .assert()
+        .success();
 }