fix(cli): keep normal help levels below ultra

The library reports an absolute maximum compression level of 22, but the CLI
advertises levels 1 through 19 unless `--ultra` is selected. The Rust frontend
used the absolute maximum for both help lines, diverging from the original C
contract and failing the help golden test. Separate the normal help ceiling
from the ultra ceiling and add a focused regression assertion.

Test Plan:
- `cargo fmt --manifest-path rust/cli/Cargo.toml -- --check` -- passed
- `cargo clippy --manifest-path rust/cli/Cargo.toml --lib -- -D warnings` -- passed
- CLI Rust tests -- 162 passed
- `make -C tests -j2 test-cli-tests` -- all 41 passed
- `git diff --check` -- passed

rust/src/zstd_cli.rs

@@ -680,7 +680,7 @@ fn program_basename(value: &OsStr) -> String {
 
 #[cfg(feature = "compression")]
 fn max_c_level_for_help() -> i32 {
-    unsafe { ZSTD_maxCLevel() }
+    DEFAULT_MAX_CLEVEL
 }
 
 #[cfg(not(feature = "compression"))]
@@ -688,6 +688,16 @@ const fn max_c_level_for_help() -> i32 {
     19
 }
 
+#[cfg(feature = "compression")]
+fn max_c_level_for_ultra() -> i32 {
+    unsafe { ZSTD_maxCLevel() }
+}
+
+#[cfg(not(feature = "compression"))]
+const fn max_c_level_for_ultra() -> i32 {
+    19
+}
+
 fn write_basic_usage<W: Write>(out: &mut W, program_name: &str) {
     let max_level = max_c_level_for_help();
     let _ = writeln!(
@@ -830,7 +840,7 @@ fn write_advanced_usage<W: Write>(out: &mut W, program_name: &str) {
     let _ = writeln!(
         out,
         "  --ultra                       Enable levels beyond 19, up to {}; requires more memory.",
-        max_c_level_for_help()
+        max_c_level_for_ultra()
     );
     let _ = writeln!(
         out,
@@ -2926,6 +2936,11 @@ mod tests {
         assert_eq!(cli.compression_format, CompressionFormat::Zstd);
     }
 
+    #[test]
+    fn normal_help_ceiling_excludes_ultra_levels() {
+        assert_eq!(max_c_level_for_help(), 19);
+    }
+
     #[test]
     fn format_option_selects_frame_type_and_suffix() {
         let cli = parse(&["zstd", "--format=gzip", "input"]);