Remove constants=None; explicit None raises with migration hint #261

Completes #260 (which ships Constants.copy() and the DeprecationWarning in 1.4). In 2.0, explicit constants=None raises TypeError naming both replacements:

• constants=Constants() for fresh library defaults
• constants=CONSTANTS.copy() for a private snapshot of current shared config

It must raise, never be reinterpreted — silently flipping None to mean "shared" would invisibly change behavior for every existing caller.

Checklist

• Explicit None raises TypeError with the migration hint
• Remove the None-constructs-Constants() branch from __init__; annotation narrows to constants: Constants = CONSTANTS
• Update customize.rst mentions and the migration section