rename gem to '3ncr', module to 'Threencr'

[peo-machine]

Summary

Renames the Ruby gem ahead of its first RubyGems publish, aligning with the brand-matching name 3ncr on every registry that allows digit-first identifiers. RubyGems' specification_policy.rb requires only that the name "include at least one letter" — 3ncr qualifies.

	Before	After
Gem name	`tokencrypt`	`3ncr`
Top-level module	`Tokencrypt`	`Threencr`
Require / install	`gem install tokencrypt; require 'tokencrypt'`	`gem install 3ncr; require '3ncr'`

Ruby constants can't start with a digit, so the gem-name vs module-name split is forced (same shape as the Python and Rust renames in 3ncr/tokencrypt-python#15 and 3ncr/tokencrypt-rust#12).

What changed

• Renamed: `tokencrypt.gemspec` → `3ncr.gemspec`; `lib/tokencrypt.rb` → `lib/3ncr.rb`; `lib/tokencrypt/version.rb` → `lib/threencr/version.rb`; `test/test_tokencrypt.rb` → `test/test_3ncr.rb`.
• Inside `3ncr.gemspec`: `spec.name = "3ncr"`; version constant references `Threencr::VERSION`; `require_relative` path updated.
• Inside `lib/3ncr.rb`: `module Tokencrypt` → `module Threencr`; nested require path updated.
• Inside `lib/threencr/version.rb`: `module Tokencrypt` → `module Threencr`.
• Tests: `require 'tokencrypt'` → `require '3ncr'`; `Tokencrypt::` → `Threencr::` throughout.
• README: title, RubyGems badge URLs, install snippet, all module references and code examples.

GitHub repo path stays `3ncr/tokencrypt-ruby`.

Test plan

• `bundle install` succeeds
• `bundle exec rake test` — 17 / 17 runs, 37 / 37 assertions pass
• `gem build 3ncr.gemspec` produces `3ncr-1.0.0.gem`
• CI green
• After merge: actual RubyGems publish (still needs operator pairing — account + MFA + first `gem push`, then OIDC Trusted Publishing)