Document limits, load control and performance

- New docs/09-limits-and-performance.md: principles, all protection
  layers, what players see, configuration reference, measurements,
  tuning guide, monitoring and how to test
- Server chapter summarises the limits and links to chapter 9; circuit
  breaker description and AI_MAX_INFLIGHT default corrected
- Architecture: new files in the layout, design decisions for load
  handling; docs index lists chapter 9
- Protocol, AI and operations chapters link to chapter 9; test counts
- Root and component READMEs describe the limits and settings

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/07-ai.md

@@ -268,7 +268,9 @@ count as a win for everyone tied). Seeded for fair comparisons.
   primary agent already is the heuristic. The reply's `agent` field says which
   one decided;
 - `OMP_NUM_THREADS` / `MKL_NUM_THREADS` are set to `AI_WORKERS` in
-  `compose.yaml`, so numeric libraries don't start a thread per host core;
+  `compose.yaml`, so numeric libraries don't start a thread per host core
+  (the whole picture, including the server side, is in
+  [chapter 9](09-limits-and-performance.md));
 - errors while deciding are reported back as `error` (the server then plays a
   random move) instead of crashing;
 - reconnects forever with exponential backoff (1 s … 30 s); logs a hint if the