chore: deny future-incompatible warnings

Carried back from Bari 206, where sixteen deferred hard errors had
accumulated unnoticed because nothing distinguished them from style
warnings in the build log.

Future-incompatibility warnings are rustc saying this code stops compiling
on some later release. Denying the group makes that a build failure the
day it appears rather than a surprise during an unrelated toolchain bump.
Every plugin cloned from this template inherits the guard.

Clippy and style lints stay warnings; only the future_incompatible group
is denied.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01WHt4DykxXaQnyaJfyS8KCn

Cargo.toml

@@ -23,6 +23,14 @@ vst3 = ["nih_plug/vst3"]
 # `cdylib` is the plugin binary; `lib` (rlib) lets `cargo test` link the unit tests.
 crate-type = ["cdylib", "lib"]
 
+# Future-incompatibility warnings are deferred hard errors: rustc is telling you
+# this code stops compiling on some later release. Denying the group turns that
+# into a build failure the day it appears, rather than accumulating unnoticed
+# until a toolchain bump breaks the build on a day you did not choose. Style and
+# clippy lints are deliberately untouched - they stay warnings.
+[lints.rust]
+future_incompatible = { level = "deny", priority = -1 }
+
 [dependencies]
 # Pinned to the same commit as musiikki/codename-206.
 # Update the `rev` when you want to update NIH-plug.