spb/forge Public MIT

- quant "int8"|"ternary": per-row fake-quant each forward (BitNet-style
  absmean for ternary), straight-through estimator backward, f32 masters;
  wired through the Linear quantization seam
- n_experts/moe_top_k/n_shared_experts: softmax router, renormalized top-k
  gates (topk_renorm + row_scale ops, CPU+Metal), differentiable
  load-balance loss, DeepSeek-style always-active shared experts;
  v1 computes experts densely (correctness first)
- qk_norm (Qwen3/Gemma3), final_softcap (Gemma2), scale_embeddings (Gemma)
- new kernels: quant.metal, moe.metal, softcap in elementwise.metal
- CPU references + parity tests for every new op and full-model variants
  (QAT int8/ternary, MoE 4+1shared, qk-norm+softcap+embed-scale)

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

A complete transformer training stack with no ML dependencies: tensors,
autograd, hand-written Metal kernels, flash attention (forward and backward),
AdamW, BPE tokenizer, checkpointing and generation. Architecture is fully
config-driven — the same binary trains 12M to 205M parameter models.

Every Metal kernel is validated against a CPU reference (85 parity checks,
<=1e-4, most bit-exact), gradients against central finite differences, and
each optimization was accepted only after the training loss trajectory stayed
numerically unchanged.

Measured findings (M5 Max, documented in RESEARCH.md and paper/forge.tex):

- `constant constexpr` for MSL tile constants declares an address-space
  variable, not a compile-time constant. Loops stop unrolling and every
  matrix accumulator spills: 0.82 -> 10.21 TFLOPS once switched to enums.
- That defect is invisible in the AIR at every -O level, because unrolling
  happens in the driver back end. Benchmark; do not read the IR.
- Register pressure, not bandwidth, dominates attention backward. Guided by
  measured spill counts, three restructurings took it 107 -> 7.05 ms (15.2x).
- On M5, mpp::tensor_ops::matmul2d reaches 51.5 TFLOPS with f16 operands vs
  10.6 for a tuned simdgroup_matrix kernel (4.9x), verified numerically.
  f16 on the simdgroup path alone is worth only +18-22%.
- Concurrent dispatch for the optimizer sweep: +22% on the 100M config.

Trained the 12.2M config for one epoch over 19.14M TinyStories tokens:
loss 8.40 -> 2.99, validation 3.009, perplexity 20.27, ~38.2k tokens/sec.