Refactor nested layout support

Problem/Motivation

Nested layout support in lb_plus relies on a nestedStoragePath string (e.g., 0/uuid/1) that encodes the traversal path to a nested section. This path must be threaded through routes, forms, JS, event subscribers, and tempstore operations across 180+ code references. It makes nested layout handling fragile, hard to extend, and tightly coupled to SectionStorageHandler — a god-object that mixes concerns like path encoding/decoding, section traversal, block content loading, and tempstore updates.

Proposed resolution

Replace the path-based system with a NestedAwareSectionStorage decorator that wraps any SectionStorageInterface and provides:

• UUID-based lookups via a TreeIndex — O(1) access to any block or section by UUID, eliminating path encoding/decoding.
• Explicit bubbling — bubbleChangesToRoot() replaces the old updateSectionStorage() pattern of manually threading changes back up.
• Scoped storage — forLayoutBlock($uuid) returns a view scoped to a nested layout block, so core forms using delta-based access work transparently with nested layouts.
• Automatic service decoration — all section storages are wrapped via service decoration, so the nested-aware API is available everywhere without manual wrapping.

This eliminates SectionStorageHandler, nestedStoragePath from routes/JS/events, and lb_plus overrides of core's ConfigureSectionForm and RemoveSectionForm. Core and contrib routes with a block {uuid} parameter work automatically with nested layouts via a NestedRouteEnhancer. Section routes use a ?section_uuid= query parameter instead of lb_plus-specific route overrides.

The edit_plus_lb and lb_plus_edit_plus bridge modules are consolidated — their functionality moves into lb_plus (registered conditionally when edit_plus is available) with update hooks to auto-uninstall the deprecated modules.

API changes

• SectionStorageHandler removed. Callers should use NestedSectionStorageInterface / NestedAwareSectionStorage instead.
• nestedStoragePath removed from routes, events, JS, and contextual link parameters. Use UUID-based methods (getPath(), getSectionsFor(), forLayoutBlock()) instead.
• SectionToolIndicatorEvent and BlockToolIndicatorEvent use layoutBlockUuid / isNested() instead of nestedStoragePath.
• Routes simplified — block operations use {uuid} only; section operations use core routes with ?section_uuid= query parameter.
• ConfigureSectionForm and RemoveSectionForm removed — core forms work via route enhancer + response subscriber.

Data model changes

None.