feat: add a new @effect decorator for reacting to signals and memos.

[trusktr]

Features:

• A new @effect decorator has been added to create reactive effects based on signals and memos. Use @effect on methods to automatically run them when their dependencies change.
• Introduced a metadata shim until native support is available, and used metadata to refactored decorators to simplify initialization and finalization processes.
• Added tests for invalid usages, and for subclassing behavior for all decorators.
• Updated signalify to apply batch() so that if a signal setter writes to a superclass signal, it doesn't cause multiple updates.
• Improved overall type definitions and metadata management for better clarity and maintainability.

BREAKING: Updated the @memo decorator to remove support for usage on class fields.

• migration: @memo foo = () => a() + b() should be changed to @memo get foo() { return a() + b(); }

[Copilot]

Pull request overview

This PR introduces a new @effect decorator for creating reactive effects declaratively on class methods, alongside significant refactoring of the decorator infrastructure to use metadata for improved initialization ordering. The changes include a metadata shim for decorator support, refactored memo/signal decorators, and performance improvements using batch() to prevent multiple effect runs.

Key Changes:

• New @effect decorator for declaring reactive effects on methods and auto-accessors
• Metadata-based decorator initialization system replacing the previous approach
• Breaking change: @memo no longer supports class fields (must use getters/methods/accessors)

Reviewed changes

Copilot reviewed 24 out of 95 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/decorators/effect.ts	New @effect decorator implementation with startEffects/stopEffects helpers
src/decorators/metadata-shim.ts	Polyfill for Symbol.metadata until native support
src/_state.ts	Refactored member tracking using metadata with custom initialization order
src/decorators/types.ts	Updated type definitions for metadata-based system
src/decorators/signal.ts	Added batch() to setters, refactored to use metadata system
src/decorators/memo.ts	Removed field support (breaking), refactored to metadata system
src/signals/signalify.ts	Wrapped setter logic in batch() to prevent multiple effect runs
src/signals/memoify.ts	Added member stat parameter, improved error handling
src/mixins/Effectful.ts	Major refactor with startEffects/clearEffects, improved lifecycle
src/decorators/effect.test.ts	Comprehensive tests for @effect decorator
src/mixins/Effectful.test.ts	New test file for Effectful mixin
src/decorators/signal.test.ts	Added subclassing and invalid usage tests
src/decorators/memo.test.ts	Updated tests for breaking changes, added subclassing tests
src/index.test.ts	Added custom element test classes
src/example.ts	Updated example to demonstrate @effect usage
src/decorators/component.ts	Added ref support and improved lifecycle

The implementation is well-tested with comprehensive coverage of edge cases, invalid usages, and subclassing behavior. The breaking change to @memo is clearly documented with migration guidance.

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