feat(SAC_LSTM_Bot): mirror-twin sparring partner + readiness check (#54)

- make_twin.sh: generates self-contained SacTwin dir in the sample-bots
  archive (own json/sh identity, own weights dir seeded from a frozen
  sac_best.zip copy, own round_counter) so RunTraining.java resolves it
  like any sample bot; re-running resets the twin to the frozen baseline.
- src/SAC_LSTM_Bot.nim: SACLSTM_BOT_JSON env overrides the baked-in bot
  json (loadBotInfo gives json total precedence, #49) so the same binary
  boots under the twin's name.
- sac_train.sh: chunk loop is a while, not for-over-seq — a crash on the
  FINAL chunk previously fell through ((chunk--);continue on an exhausted
  seq list) and exited 0 with budget incomplete; observed live vs SacTwin.

Readiness dry-run (#54): weighted pool Corners:1,SacTwin:3 picked the twin
in 3/4 chunks; all battles counter-checked; deterministic eval parsed;
main sac_best.zip/counter untouched by twin (twin counter advanced
independently); crash-restart proven end-to-end incl. final-chunk retry.

SAC_LSTM_Bot/src/SAC_LSTM_Bot.nim

@@ -17,7 +17,11 @@ import SAC_LSTM_Bot/actions
 import SAC_LSTM_Bot/rewards
 import SAC_LSTM_Bot/integration
 
-const botJsonPath = currentSourcePath().parentDir / "SAC_LSTM_Bot.json"
+# Identity json: baked-in src json by default; SACLSTM_BOT_JSON lets a mirror
+# twin (#54) boot the same binary under its own name (loadBotInfo gives the
+# json total precedence over env, so the twin must point at its own file).
+let botJsonPath = getEnv("SACLSTM_BOT_JSON",
+    currentSourcePath().parentDir / "SAC_LSTM_Bot.json")
 
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