Implement named slots with ordering for structure entries

Acceptance Criteria

• RuneConfig in packages/transform/src/types.ts has slots?: string[]
• StructureEntry has slot?: string and order?: number fields
• When slots is declared, the engine assembles children by iterating slots in order, collecting structure entries per slot sorted by order, and placing content children at the 'content' slot
• When slots is not declared, existing before/after behavior is unchanged (backward compatible)
• When slots is declared, before: true maps to the first non-content slot and before: false maps to the last non-content slot
• Explicit slot assignments take precedence over before mapping
• mergeThemeConfig merges slots arrays (theme replaces base) and structure entries with slot assignments
• Unit tests cover: slot assembly ordering, order within slots, content slot placement, backward compat without slots, before-to-slot mapping, merge behavior
• TypeScript compiles cleanly across all packages
• All existing tests pass

Approach

1. Add interface changes to packages/transform/src/types.ts
2. Modify the engine's child assembly logic in packages/transform/src/engine.ts to support slot-based ordering when slots is present
3. Implement the backward-compat mapping from before to slot positions
4. Update mergeThemeConfig in packages/transform/src/merge.ts
5. Write comprehensive tests for slot assembly, ordering, and backward compat

References

• Structure Slots and Declarative Flexibility (Feature 1 — Named Slots with Ordering)
• Add value mapping to modifier config (value mapping should land first)
• Replace hardcoded density contexts with configurable childDensity (density contexts should land first)