Add Module#private_constants block for concise private constants

[mliem2k]

Summary

Constants don't respect private/public regions the way methods do, so making one private normally means repeating its name in a separate private_constant call:

class Configuration
  DEFAULT_TIMEOUT = 30
  private_constant :DEFAULT_TIMEOUT
end

This implements the private_constants do ... end API suggested in the issue: every constant defined directly within the block is automatically marked private via private_constant once the block finishes.

class Configuration
  private_constants do
    DEFAULT_TIMEOUT = 30
  end
end

Configuration::DEFAULT_TIMEOUT # ❌ NameError: private constant Configuration::DEFAULT_TIMEOUT referenced

It only marks constants introduced during the block (diffing constants(false) before/after), so pre-existing constants are left untouched, and it's a no-op if the block doesn't define any new constants (avoiding Ruby's "private_constant with no argument is just ignored" warning).

Added to lib/type_toolkit/ext/module.rb, next to interface!, since it's a similar small Module-level extension. Also added a README section under "Tools".

Test plan

• bundle exec rake test — 95 runs, 0 failures
• bundle exec rake rubocop — no offenses
• bundle exec rake typecheck / typecheck_prism — no errors
• New specs in spec/private_constants_spec.rb cover: constants defined via = and via class/module keywords being made private, constants outside the block staying public, unqualified access still working from the defining scope, a no-op empty block, and exceptions inside the block propagating normally

Fixes #9

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