dev: implement PSR-4 and reduce Singleton usage

[justlevine]

What

This PR begins the process of code standardization/sharing. Specifically it:

• Standardizes the following files from other One* plugins:
  • uninstall.php
  • onedesign.php
  • inc/Autoloader.php
  • inc/Main.php
  • inc/Contracts/Interfaces/Registrable.php
  • inc/Contracts/Traits/Singleton.php
  • inc/Modules/Rest/Abstract_Rest_Controller.php
  • inc/Modules/Rest/Rest.php
• Moves classes into the PSR-4 namespace, (guestimated to prevent merge conflicts, but not final)
• Replaces undesired usages of Singleton, with class instantiation and moves hook registration out of the constructor.

Why

This should prevent the bulk of potential merge conflicts, making it easier for future PRs to be independent and reviewable in isolation.

Related Issue(s):

• Part of https://github.com/rtCamp/OnePress/issues/49

Additional Info

• Docs etc have not been updated, since there are future incoming changes.
• The part the requires detailed review are the "standardized" files. These will be the source of truth going forward. Any changes beyond those are just a result of the new patterns, and those classes will likely be standardized/reviewed in follow-up PRs.

Checklist

• I have read the Contribution Guidelines.
• I have read the Development Guidelines.
• My code is tested to the best of my abilities.
• My code passes all lints (ESLint etc.).
• My code has detailed inline documentation.
• I have updated the project documentation as needed.

[Copilot]

Pull request overview

This PR implements PSR-4 autoloading and reduces singleton usage across the OneDesign plugin. It standardizes file structure and naming conventions to align with other One* plugins.

Key changes:

• Introduces a Registrable interface and refactored Singleton trait for better separation of concerns
• Replaces singleton pattern with explicit class instantiation in most modules
• Moves classes into PSR-4 compliant namespaces (e.g., OneDesign\Modules\Rest, OneDesign\Modules\Post_Types)

Reviewed changes

Copilot reviewed 31 out of 32 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
uninstall.php	Refactored to use namespaced functions and improved multisite handling
onedesign.php	Updated to use new Main::instance() instead of Plugin::get_instance()
inc/Main.php	New bootstrap class that instantiates and registers all plugin components
inc/Contracts/Traits/Singleton.php	New simplified singleton trait with improved error handling
inc/Contracts/Interfaces/Registrable.php	New interface for classes that register WordPress hooks
inc/Modules/Rest/Abstract_REST_Controller.php	New base class for REST controllers with shared namespace
inc/Modules/Rest/*.php	Refactored REST controllers to extend abstract base and implement Registrable
inc/Modules/Post_Types/*.php	Updated to use PSR-4 namespaces and Registrable interface
inc/Modules/Core/Assets.php	Removed Singleton, now instantiated directly
inc/Modules/Settings/Admin.php	Moved to proper namespace and implements Registrable
inc/Modules/Multisite/Multisite.php	Updated namespace and implements Registrable
custom-functions.php	New file with global helper functions moved from inc/helpers/
Configuration files	Updated to include custom-functions.php and ignore generic type warnings

Comments suppressed due to low confidence (1)

inc/Modules/Core/Assets.php:31

• The build_localized_data() method is declared as static (line 47), but is being called as an instance method using $this->. This should be self::build_localized_data() or the method should not be static.

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[up1512001]

Added few suggestions.

[up1512001]

LGTM