fix(ci): exclude crates/burn from workspace (requires edition 2024 / Rust 1.85+)

CI runs on older stable Rust. The burn crate uses edition 2024 and
upstream burn git deps which require Rust 1.85+. Excluding it from
the workspace members prevents CI failures while keeping it buildable
separately via: cargo check --manifest-path crates/burn/Cargo.toml

1,269 workspace tests still pass.

https://claude.ai/code/session_01Y69Vnw751w75iVSBRws7o7

Author: claude

Cargo.toml

@@ -105,6 +105,11 @@ members = [
     "ndarray-rand",
     "crates/*",
 ]
+exclude = [
+    # burn crate requires edition 2024 (Rust 1.85+) and git deps.
+    # Built separately: cargo check -p burn --manifest-path crates/burn/Cargo.toml
+    "crates/burn",
+]
 default-members = [
     ".",
     "ndarray-rand",