🚀 feat: Multiple improvements for Nyash parser and LLVM backend

Parser improvements:
- Added expression statement fallback in parse_statement() for flexible syntax
- Fixed ternary operator to use PeekExpr instead of If AST (better lowering)
- Added peek_token() check to avoid ?/?: operator conflicts

LLVM Python improvements:
- Added optional ESC_JSON_FIX environment flag for string concatenation
- Improved PHI generation with better default handling
- Enhanced substring tracking for esc_json pattern

Documentation updates:
- Updated language guide with peek expression examples
- Added box theory diagrams to Phase 15 planning
- Clarified peek vs when syntax differences

These changes enable cleaner parser implementation for self-hosting,
especially for handling digit conversion with peek expressions instead
of 19-line if-else chains.

🤖 Generated with [Claude Code](https://claude.ai/code)

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

tools/ny_parser_mvp_roundtrip.sh

@@ -0,0 +1,32 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+SCRIPT_DIR=$(CDPATH= cd -- "$(dirname -- "$0")" && pwd)
+ROOT_DIR=$(CDPATH= cd -- "$SCRIPT_DIR/.." && pwd)
+BIN="$ROOT_DIR/target/release/nyash"
+
+if [[ ! -x "$BIN" ]]; then
+  echo "[build] nyash (release) ..." >&2
+  cargo build --release >/dev/null
+fi
+
+mkdir -p "$ROOT_DIR/tmp"
+TMP_SRC="$ROOT_DIR/tmp/ny_parser_input.ny"
+printf 'return 1+2*3\n' > "$TMP_SRC"
+
+# Run parser MVP (Python) to JSON v0, then pipe to bridge
+JSON_OUT=$(python3 "$ROOT_DIR/tools/ny_parser_mvp.py" "$TMP_SRC")
+if [[ -z "$JSON_OUT" ]]; then
+  echo "error: parser produced no JSON" >&2
+  exit 1
+fi
+echo "$JSON_OUT" | "$BIN" --ny-parser-pipe >/tmp/ny_parser_mvp_rt.out || true
+
+if rg -q '^Result:\s*7\b' /tmp/ny_parser_mvp_rt.out; then
+  echo "✅ Ny parser MVP roundtrip OK" >&2
+  exit 0
+else
+  echo "❌ Ny parser MVP roundtrip FAILED" >&2
+  cat /tmp/ny_parser_mvp_rt.out >&2 || true
+  exit 2
+fi