fix(compiler): pop stale bool from stack in emitLoopBackwards (#954)

emitLoopBackwards uses OpMoreOrEqual + OpJumpIfFalse to check the loop
condition, but never pops the comparison result from the stack. This
leaves a stale bool that corrupts the value stack for the parent context.

When findLast or findLastIndex is used inside a map literal like
{"r": findLast([1, 2, 3], # > 3)}, the OpMap opcode tries to pop a
string key but finds the leftover bool instead, causing:
  interface conversion: interface {} is bool, not string

The forward-iterating emitLoop avoids this by using OpJumpIfEnd, which
checks scope.Index directly without touching the stack.

Fix: add OpPop after OpJumpIfFalse (continue path) and after patchJump
(exit path), matching the convention used by emitCond and every other
OpJumpIfFalse callsite in the compiler.

Fixes #950

Co-authored-by: lawrence3699 <lawrence3699@users.noreply.github.com>

Author: lawrence3699

compiler/compiler.go

@@ -1213,12 +1213,14 @@ func (c *compiler) emitLoopBackwards(body func()) {
 	c.emit(OpInt, 0)
 	c.emit(OpMoreOrEqual)
 	end := c.emit(OpJumpIfFalse, placeholder)
+	c.emit(OpPop)
 
 	body()
 
 	c.emit(OpDecrementIndex)
 	c.emit(OpJumpBackward, c.calcBackwardJump(begin))
 	c.patchJump(end)
+	c.emit(OpPop)
 }
 
 func (c *compiler) PredicateNode(node *ast.PredicateNode) {

expr_test.go

@@ -1275,6 +1275,22 @@ func TestExpr(t *testing.T) {
 			`findLastIndex(1..9, # % 2 == 0)`,
 			7,
 		},
+		{
+			`{"r": findLast([1, 2, 3, 4, 5], # > 3)}`,
+			map[string]any{"r": 5},
+		},
+		{
+			`{"r": findLastIndex([1, 2, 3, 4, 5], # > 3)}`,
+			map[string]any{"r": 4},
+		},
+		{
+			`{"r": findLast([1, 2, 3], # > 10)}`,
+			map[string]any{"r": nil},
+		},
+		{
+			`{"a": findLast([1, 2, 3], # > 1), "b": findLastIndex([4, 5, 6], # > 4)}`,
+			map[string]any{"a": 3, "b": 2},
+		},
 		{
 			`filter(1..9, # % 2 == 0)[-1]`,
 			8,