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/make_twin.sh

@@ -0,0 +1,56 @@
+#!/usr/bin/env bash
+# make_twin.sh — #54 mirror-twin sparring partner generator.
+#
+# Builds a self-contained `SacTwin` bot dir inside the sample-bots archive so
+# RunTraining.java resolves it like any sample bot ($SAMPLE_BOTS_DIR/<name>).
+# The twin is the SAME binary as SAC_LSTM_Bot but with:
+#   - distinct identity  (SacTwin.json; SACLSTM_BOT_JSON overrides the baked-in
+#     src json — loadBotInfo gives json total precedence over env, #49 lesson)
+#   - its OWN weights dir, seeded from a FROZEN copy of weights/sac_best.zip
+#     (no checkpoint write races with the main bot)
+#   - its OWN round_counter.txt (main liveness guard untouched)
+#
+# Re-running resets the twin to the frozen baseline (reproducible opponent).
+#
+# Usage: ./make_twin.sh [target-archive-dir]
+set -euo pipefail
+
+SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd)"
+TARGET="${1:-${SAMPLE_BOTS_DIR:-/home/davide/Projects/tank-royale/sample-bots/java/build/archive}}/SacTwin"
+BIN="$SCRIPT_DIR/SAC_LSTM_Bot"          # nimble build -d:release output
+SEED="$SCRIPT_DIR/weights/sac_best.zip" # frozen baseline
+
+[ -x "$BIN" ]   || { echo ">>> $BIN missing — run 'nimble build -d:release' first"; exit 1; }
+[ -f "$SEED" ]  || { echo ">>> $SEED missing — need at least one eval'd checkpoint"; exit 1; }
+
+mkdir -p "$TARGET/weights"
+cp "$BIN" "$TARGET/SAC_LSTM_Bot"
+
+cat > "$TARGET/SacTwin.json" <<EOF
+{
+  "name": "SacTwin",
+  "version": "0.1.0",
+  "authors": ["Davide Cappellini"],
+  "description": "Mirror-twin sparring partner of SAC_LSTM_Bot (#54), generated by make_twin.sh — own weights dir, frozen seed",
+  "homepage": "",
+  "countryCodes": ["IT"],
+  "gameTypes": ["classic", "melee", "1v1"],
+  "platform": "Nim",
+  "programmingLang": "Nim"
+}
+EOF
+
+cat > "$TARGET/SacTwin.sh" <<'EOF'
+#!/bin/sh
+# Twin launcher (#54): identity + weights fully decoupled from the main bot.
+DIR="$(cd "$(dirname "$0")" && pwd)"
+export SACLSTM_BOT_JSON="$DIR/SacTwin.json"
+export SACLSTM_WEIGHTS_PATH="$DIR/weights/sac_latest.zip"
+exec "$DIR/SAC_LSTM_Bot"
+EOF
+chmod +x "$TARGET/SacTwin.sh" "$TARGET/SAC_LSTM_Bot"
+
+cp "$SEED" "$TARGET/weights/sac_latest.zip"
+cp "$SEED" "$TARGET/weights/sac_best.zip"
+echo 0 > "$TARGET/weights/round_counter.txt"
+echo ">>> twin ready: $TARGET (seeded from $(stat -c %y "$SEED" | cut -d. -f1) snapshot of sac_best.zip)"