Implement element projection (hide, group, relocate)

Acceptance Criteria

• RuneConfig in packages/transform/src/types.ts has projection?: { relocate?, group?, hide? } with the interfaces from SPEC-033
• The engine implements a projection pass between Phase 6 (recursion/BEM) and Phase 7 (meta filtering)
• Hide: elements matching hide entries are removed from the children array entirely
• Group: elements matching members are collected, removed from current positions, wrapped in a new container with data-name set to the group key, and placed at the first collected member's position (or into a named slot)
• Relocate: elements found by data-name are moved into the target (another data-name element or a slot name) at the specified position (prepend/append)
• Execution order is hide → group → relocate (so groups can be relocation targets)
• Group wrappers get BEM element classes via normal applyBemClasses flow
• mergeThemeConfig handles projection (theme projection fully replaces base, not deep-merged)
• Unit tests cover: hide removes element, group collects and wraps, relocate moves into target, relocate into slot, combined hide+group+relocate, invalid data-name references are no-ops
• TypeScript compiles cleanly
• All existing tests pass

Approach

1. Add projection interfaces to types
2. Implement the projection pass as a separate function in the engine, called between Phase 6 and Phase 7
3. Each sub-pass (hide, group, relocate) walks the children array and applies its transformation
4. For relocate targeting slots, integrate with the slot assembly from WORK-112
5. Update merge logic
6. Write comprehensive tests for each operation and their composition

References

• Structure Slots and Declarative Flexibility (Feature 5 — Element Projection)
• Implement named slots with ordering for structure entries (slots — projection relocate can target slot names)