Fix 10 confirmed bugs across evaluator, object, scanner, and library

[kaidesu]

• return inside for/for-in/while loops was silently swallowed: isTerminator
  did not include RETURN, so *object.Return fell through and iteration
  continued instead of propagating the return value up to the function call

• compound assignment (+=, -=, etc.) panicked on index/property LHS because
  evaluateCompound unconditionally cast node.Left to *ast.Identifier; now
  delegates to evaluateIndexAssignment / evaluatePropertyAssignment

• evaluateAssign dropped the error for unrecognised assignment targets by
  calling object.NewError without returning it

• list index assignment panicked on non-number index (e.g. list["x"] = 1)
  due to unchecked type assertion; now returns a runtime error

• division and modulo by zero: integer % 0 caused a Go runtime panic;
  float division silently produced +Inf; both now return a runtime error

• string.find() used found[1] (first capture group) instead of found[0]
  (full match), panicking on regexes with no capture groups

• list.first() and list.last() panicked with index out of range on empty
  lists; they now return null consistent with other list methods

• scanner.scanString did not process backslash escape sequences, so \n \t
  \r \ " ' were stored as two-character literals in string tokens

• math.max and math.min called panic() on bad arity instead of returning
  a structured runtime error object

• trait method lookup only checked receiver.Class.Traits; now walks the
  full superclass chain so traits on parent classes are visible to children

https://claude.ai/code/session_0136eEhe68sB3Mi22WwDLhrB