hdr: gate hdr_rolling_floor_bench on std like the other hpc examples

The no-default-features test build compiles every example; hpc exists
only under std.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019HnekoM1EidTwQLS3oFVFm

Author: claude

Cargo.toml

@@ -207,6 +207,10 @@ required-features = ["std"]
 name = "rolling_floor_probe"
 required-features = ["std"]
 
+[[example]]
+name = "hdr_rolling_floor_bench"
+required-features = ["std"]
+
 [[example]]
 name = "codec_overlap_probe"
 required-features = ["std"]