docs: fix documentation errors across README and assets/docs

- Correct training CLI args: remove non-existent --adamw_beta1/2, fix --weight_decay
- Fix optimizer section: document MuonMix instead of plain AdamW
- Fix inference.md decode phase description (all groups, not largest)
- Fix dataflow.md H5Store description (no share_memory_ in code)
- Fix architecture.md class diagram: remove Task.stream_callback,
  EncoderConfig.use_gated_attention; update stop_ids docs
- Fix --min_rate default value description to match code (0.01)
- Update all document timestamps to 2026-07-09

assets/docs/training.md

@@ -201,9 +201,7 @@ nohup python scripts/tools/train.py \
     --warmup_ratio=0.05 \
     --max_lr=1e-4 \
     --max_grad_norm=1.0 \
-    --adamw_beta1=0.9 \
-    --adamw_beta2=0.95 \
-    --adamw_weight_decay=0.01 \
+    --weight_decay=0.1 \
     --window_size=2048 \
     --ckpt_interval=10000 \
     --ckpt_dir=./checkpoint \
@@ -214,4 +212,4 @@ nohup python scripts/tools/train.py \
 
 Full parameter reference at [params.md](params.md).
 
-> Document Update Time: 2026-07-05
+> Document Update Time: 2026-07-09