Cargo: gate codec_mode_histogram example behind required-features=["codec"]

Addresses codex P1 (#229): the example imports hpc::codec, which is not a
default feature. Without a [[example]] required-features entry, default
cargo test (which builds examples) would try to compile it without --features
codec and fail resolving hpc::codec. Now default builds skip the target;
--features codec builds it. Matches the sibling-example convention.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MLBnPuScZy6w9di2QEjsXM

Author: claude

Cargo.toml

@@ -79,6 +79,10 @@ required-features = ["std"]
 name = "edge_codec_compare"
 required-features = ["std"]
 
+[[example]]
+name = "codec_mode_histogram"
+required-features = ["codec"]
+
 [[example]]
 name = "entropy_ladder_probe"
 required-features = ["std"]