refactor(burn): symlink overlay — 15K upstream lines → 35 symlinks + 2 files Replace 11,775 lines of copied upstream burn-ndarray source with: - Git submodule: crates/burn/upstream/ (pinned at ed72d2b) - 35 symlinks: crates/burn/src/*.rs → upstream/crates/burn-ndarray/src/ - 2 real files: ops/tensor.rs (our SIMD wires), ops/activation.rs (fused sigmoid) Our total owned code: 846 lines (801 tensor + 45 activation). Upstream code: 0 lines tracked (lives in submodule). The repo shrinks by ~11K lines while keeping the same functionality. Build: cargo check --manifest-path crates/burn/Cargo.toml Test: cargo test --manifest-path crates/burn/Cargo.toml (30 pass) https://claude.ai/code/session_01Y69Vnw751w75iVSBRws7o7#41