feat(encryption): add a shared Encryption service over the framework Encryptor

[aryanjasala]

What this does

Wires the theme into the framework's instance-based Encryptor, following the
same consumer pattern as Assets / Components / Templates: a context-owned
Core service shared through the container, with Util wrappers as the call
surface.

Changes

• Add inc/Core/Encryption.php — final class Encryption extends Encryptor implements Shareable; overrides the key() seam to source the key from the
  ELEMENTARY_ENCRYPTION_KEY constant. No salt fallback — without the constant
  the framework's parent::key() throws on first use.
• inc/Main.php — register Encryption in CLASSES.
• inc/Helpers/Util.php — Util::encrypt() / Util::decrypt() backed by a
  shared encryptor() resolved via get_shared( Encryption::class ).

Configuration

Define a dedicated key in wp-config.php before first use:

define( 'ELEMENTARY_ENCRYPTION_KEY', '<32+ bytes of random data>' );

There is deliberately no LOGGED_IN_KEY fallback: an auth salt should not
double as an encryption key, and rotating WordPress salts (standard incident
response) must never invalidate encrypted data. The throw is lazy — boot and
container registration never error; only an actual Util::encrypt() /
Util::decrypt() call without the constant does.

How I verified

• php -l clean on changed files.
• Smoke test (framework branch symlinked): Encryption extends Encryptor, is
  Shareable; without the constant the first encrypt throws the framework's
  RuntimeException; with the constant a full encrypt → decrypt roundtrip
  succeeds.

Reviewer notes

• Stacks on the framework Encryptor PR; bump composer.lock once it lands on
  main.

[Copilot]

Pull request overview

This PR introduces a shared theme-level encryption service built on the framework Encryptor, registers it in the theme bootstrap, and exposes encryption/decryption via Helpers\Util for consistent consumption across the theme.

Changes:

• Added inc/Core/Encryption.php as a shared (container-managed) encryptor service sourcing its key from ELEMENTARY_ENCRYPTION_KEY.
• Registered the new Encryption service in Main::CLASSES so it’s loaded by the theme bootstrap.
• Added Util::encrypt() / Util::decrypt() wrappers that resolve and delegate to the shared Encryption instance.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
inc/Main.php	Registers the new Core\Encryption service in the theme’s load list.
inc/Helpers/Util.php	Adds encrypt()/decrypt() wrappers backed by the shared encryptor instance.
inc/Core/Encryption.php	Implements the shared encryptor service and overrides key resolution via ELEMENTARY_ENCRYPTION_KEY.