wip: copy burn-backend + burn-std + burn-ir into ndarray (Cargo.toml rewrite pending)

Copied 3 upstream burn crates (~23K lines) to eliminate external deps.
burn/Cargo.toml now points to local path deps instead of git.
All 4 burn crates excluded from workspace (edition 2024).

PENDING: Rewrite Cargo.toml for burn-backend, burn-std, burn-ir to
resolve workspace = true refs. These crates depend on cubecl, cubecl-common,
cubecl-zspace which are GPU compute abstractions — need to determine
which are actually needed for CPU-only operation and which can be stubbed.

This is work-in-progress. The workspace (ndarray core) is unaffected:
1,269 tests pass. The burn crate compiles separately when pointed at
upstream git deps.

https://claude.ai/code/session_01Y69Vnw751w75iVSBRws7o7

Author: claude

Cargo.toml

@@ -106,9 +106,12 @@ members = [
     "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
+    # burn crates require edition 2024 (Rust 1.85+).
+    # Built separately: cargo check --manifest-path crates/burn/Cargo.toml
     "crates/burn",
+    "crates/burn-backend",
+    "crates/burn-std",
+    "crates/burn-ir",
 ]
 default-members = [
     ".",

crates/burn-backend/Cargo.toml

@@ -0,0 +1,55 @@
+[package]
+authors = ["nathanielsimard <nathaniel.simard.42@gmail.com>"]
+categories = ["science", "no-std", "embedded", "wasm"]
+description = "Core backend interfaces and data structures for executing tensor operations in Burn."
+documentation = "https://docs.rs/burn-backend"
+edition.workspace = true
+keywords = ["deep-learning", "machine-learning", "tensor", "pytorch", "ndarray"]
+license.workspace = true
+name = "burn-backend"
+readme.workspace = true
+repository = "https://github.com/tracel-ai/burn/tree/main/crates/burn-backend"
+version.workspace = true
+
+[lints]
+workspace = true
+
+[features]
+default = ["std"]
+doc = ["default"]
+std = ["rand/std", "num-traits/std", "burn-std/std", "cubecl?/std"]
+
+tracing = ["burn-std/tracing", "cubecl/tracing"]
+
+# For DTypeUsage de/serialization
+serde = ["enumset/serde"]
+
+cubecl = ["dep:cubecl", "burn-std/cubecl"]
+cubecl-cuda = ["cubecl", "cubecl/cuda"]
+cubecl-hip = ["cubecl", "cubecl/hip"]
+cubecl-wgpu = ["cubecl", "cubecl/wgpu"]
+cubecl-cpu = ["cubecl", "cubecl/cpu"]
+
+[dependencies]
+burn-std = { path = "../burn-std", version = "=0.21.0-pre.2", default-features = false }
+cubecl = { workspace = true, optional = true, default-features = false }
+
+bytemuck = { workspace = true, features = ["extern_crate_alloc"] }
+derive-new = { workspace = true }
+enumset = { workspace = true }
+hashbrown = { workspace = true }
+num-traits = { workspace = true }
+rand = { workspace = true, default-features = false }
+rand_distr = { workspace = true }
+serde = { workspace = true }
+thiserror = { workspace = true }
+spin = { workspace = true }
+
+[target.'cfg(not(target_has_atomic = "ptr"))'.dependencies]
+portable-atomic-util = { workspace = true }
+
+[dev-dependencies]
+rand = { workspace = true, features = ["thread_rng"] }
+paste = { workspace = true }
+serde_json = { workspace = true, features = ["alloc"]}
+serial_test = { workspace = true }