feat: Component slots

[stravo1]

Summary

Adds component slots so saved components can keep their internal block order locked while still allowing page authors to inject editable content into declared slot areas.

This introduces a Vue/React-style slot model for Builder components:

• Component authors can mark eligible container blocks as named slots.
• Page instances can edit content inside those slots without unlocking the rest of the component structure.
• Slot content is treated as detached instance content, so it can be deleted/reordered independently from component-owned blocks.
• Empty filled slots intentionally stay empty; deleting filled slot content does not restore fallback automatically.

Changes

Component slot authoring

• Added slot metadata to blocks: slotName and slotFilled.
• Added a block property section for declaring named component slots.
• Validates component slots server-side:
  • slot names must be unique
  • slots cannot be nested
  • slots cannot live inside repeaters
  • slots must be declared on supported container blocks

Slot behavior in component instances

• Slot fallback children are materialized as detached instance blocks.
• Blocks inside slots can be edited, deleted, duplicated, wrapped, and reordered independently.
• Component-owned blocks outside slots remain protected from structure/order changes.
• Filled slots are preserved by slot name when components are updated or synced.
• Empty filled slots remain empty instead of restoring fallback content.
• Added a “Reset Slot to Fallback” action for slot instances.

Block layers UX

• Slot blocks now use a slot-style icon prefix instead of a visible tag.
• Drag/drop restrictions now respect the distinction between component-owned blocks and detached slot content.

Component sync/update behavior

• Component sync preserves filled slot content where the slot name still exists.
• Generic warning is shown before component updates/syncs because slot removal/renaming may discard detached slot content.

Demo - Popover Component with a slot named body:

Screen.Recording.2026-06-29.at.9.19.14.PM.mp4

Assigning slots

[codecov]

Codecov Report

❌ Patch coverage is 79.72973% with 30 lines in your changes missing coverage. Please review.
✅ Project coverage is 56.64%. Comparing base (6229a87) to head (74c4bed).
⚠️ Report is 1 commits behind head on develop.

Files with missing lines	Patch %	Lines
...ilder/builder/doctype/builder_page/builder_page.py	20.00%	20 Missing ⚠️
...der/doctype/builder_component/builder_component.py	87.50%	10 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #648      +/-   ##
===========================================
+ Coverage    55.13%   56.64%   +1.51%     
===========================================
  Files           34       34              
  Lines         4001     4143     +142     
===========================================
+ Hits          2206     2347     +141     
- Misses        1795     1796       +1     

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