refactor: vendor import burn deps — pin upstream at rev, only override our additions

Revert the 23K-line copy of burn-backend/burn-std/burn-ir.
Instead: pin upstream burn at commit ed72d2b via git rev deps.

Our changes are ONLY in crates/burn/src/ (60 lines of additions):
  ops/tensor.rs:     try_vml_unary() + 4 SIMD wires (exp, log, sqrt, abs)
  ops/activation.rs: fused sigmoid via hpc::activations::sigmoid_f32

Everything else is unmodified upstream burn-ndarray source.
Upstream deps stay upstream. We only own our additions.

30 burn tests pass. 1,269 workspace tests pass.

https://claude.ai/code/session_01Y69Vnw751w75iVSBRws7o7

Author: claude

Cargo.toml

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