feat: Custom Slot - fragment-backed reusable content with instance-local fields

[Yxnnxck]

Description

This PR introduces a new core component: Custom Slot, enabling reusable synced content with per-instance field values.

It builds on top of:

• Webstudio Components #2529 (Webstudio Components)
• Slots #1116 (Slots)
• 📦 Components #2646 (Component Editor)

The goal is to bridge the gap between current Slots and a full component system by introducing:

• explicit structure (Fragment-backed root)
• instance-local values
• shared schema
• scoped data injection

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Summary

Custom Slot allows creating reusable content blocks that behave like real components:

• shared structure
• per-instance configurable fields
• isolated scope
• predictable rendering

Unlike current Slots, this removes implicit structure (e.g. "first child is special") and introduces a proper internal model.

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Motivation

Current limitations:

• Slots only support structure reuse, but not proper configuration
• No clear component abstraction (Webstudio Components #2529)
• No way to define per-instance fields
• Reuse patterns are fragile or implicit

This PR moves Webstudio toward a real component model.

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What changed

Implemented

1. New core component: ws:customSlot

• Core metas + templates added
• Insertable in project
• SDK helpers introduced

2. Data model redesign

Invariant:

Custom Slot -> Fragment (internal)

• Slot instance:
  • owns __customSlotValues
• Fragment root:
  • owns __customSlotSchema
• Derived component object

3. Rendering & traversal

• Fragment used as hidden structural root
• Navigator only reacts to visible children
• Copy/paste + tree logic updated

4. Local fields system

• Named fields defined in Settings panel
• Shared schema, local values
• UI supports:
  • inline editing
  • binding
  • rename/delete

5. Structured bindable values

• Expression-based values (not just strings)
• Evaluated in owner scope
• Dynamic component object creation

6. Scope isolation

• Slot owner → has outer scope access
• Slot children → only:
  • global scope
  • injected component

7. UX cleanup

Internal variables hidden from:

• settings scopes
• page scopes
• resource scopes
• binding popovers

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8. Not implemented

• UI polish for field management
• better discoverability in builder
• integration with future Component Editor (📦 Components #2646)
• potential promotion of Custom Slot → full component abstraction (Webstudio Components #2529)

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Why this approach

• Introduces explicit content root
• Keeps reusable structure stable
• Enables component-like mental model
• Avoids leaking internal implementation details

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Steps for reproduction

1. Insert Custom Slot
2. Add multiple children inside
3. Define fields in Settings panel
4. Create multiple instances
5. Change field values per instance

Expected:

• structure stays synced
• values differ per instance

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Code Review

• hi @kof, I need you to do
  • conceptual review (architecture, feature-correctness)
  • detailed review (read every line)
  • test it on preview

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Before requesting a review

• made a self-review
• added inline comments where things may be not obvious
  (I didn't add Comments if needed I can do so but I hope Code is clear)

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Before merging

• tested locally and on preview environment (preview dev login: 0000)
• updated test cases document
• added tests
• env variables updated (if needed)