docs: Phase 14 design doc + benchmark, fix Phase 11/12 honesty

Phase 14 (Flash Attention):
- Design doc: FA2 algorithm, SM120 hardware constraints (FA4 incompatible),
  kernel config (BR=BC=64, 32KB smem), GQA mapping, causal tile-skip,
  known limitations and optimization roadmap
- Benchmark doc: correctness (9/10 top-1 match, identical to pre-FA baseline),
  performance tracking (6.9→10.3→12.9 tok/s across phases), memory savings
  analysis, remaining bottleneck breakdown

Phase 11 doc: title corrected from "Paged Attention" to "GPU-Resident KV Cache"
with explicit note that paged allocation was not implemented.

Phase 12 doc: "当前状态" updated from "未实现" to reflect actual state —
iteration-level scheduling implemented + verified (6.0x concurrent speedup),
batched GPU forward explicitly marked as not yet implemented.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docs/11-paged-attention.md

@@ -1,8 +1,10 @@
-# Phase 11: Paged Attention + KV Cache Manager — Design Document
+# Phase 11: GPU-Resident KV Cache — Design Document
+
+> **注意**: 原计划为 "Paged Attention + KV Cache Manager"，实际实现为 GPU 连续预分配 KV cache（非 paged）。Paged allocation 留待后续优化。
 
 ## Goal
 
-将 KV cache 从 CPU Vec 迁移到 GPU，使用 block-based paging 管理显存。消除每步 decode 的 CPU round-trip（当前 KV cache 最大性能瓶颈之一）。
+将 KV cache 从 CPU Vec 迁移到 GPU，消除每步 decode 的 CPU round-trip（当前 KV cache 最大性能瓶颈之一）。
 
 ## 当前问题