Improve static error for incorrect arities of subclass constructors

Classes with private immutable fields are currently required to have custom constructors. For example,

#lang rhombus
class Secret(private _secret)

this triggers “class needs a custom constructor to initialize private immutable fields”, while

#lang rhombus
class Secret(private _secret):
  constructor (secr):
    super(secr)

this works. However, this requirement doesn’t seem to properly propagate. For example,

#lang rhombus
class Secret(private _secret):
  nonfinal
  constructor (secr):
    super(secr)

class TopSecret():
  extends Secret

this does not trigger “class needs a custom constructor to initialize private immutable fields”, but it simply cannot work.

Reproduce the two Rhombus class examples from the issue and compare the diagnostics for direct and indirect subclass construction. Trace where constructor arity and private immutable-field requirements are checked; done means the indirect subclass reports that a custom constructor is required, with regression coverage for the shown examples.