feat: add Assembler and AttributeFactory for spec attribute collection

[DerManoMann]

Summary

Introduces the assembly stage of the spec pipeline — the step that scans PHP classes and collects spec attributes into a Specification container.

Source files → ReflectionClass → Assembler → Specification

Depends on zircote#2054.

Design

Two-pass assembly

The Assembler runs two distinct passes per class:

1. Sibling merge — attributes on the same reflector compose via merge() maps. E.g., Schema merges into Property filling its $schema slot.
2. Hierarchy resolution — attributes from inner reflectors (properties, methods, parameters, constants) flow up into enclosing-level attributes via contains() maps. E.g., Operation on
   a method absorbs Parameter from its parameters; Schema on a class absorbs Property from its members.

After both passes, only root attributes remain and are added to the Specification. Non-root attributes that survive resolution trigger an error (orphan detection).

AttributeFactory

Extracted helper that encapsulates:

• Reading PHP 8.1+ attributes from reflectors
• Sibling composition via merge() slot maps (same-reflector nesting)
• Hierarchical absorption via contains() slot maps (inner → outer)

Shared between the Assembler and future augmenter pipes that need to manufacture spec objects from reflection (e.g. ExpandHierarchy for non-schema ancestor members).

Slot semantics

The merge() and contains() maps return [TargetClass => 'slotName']:

• 'parameters[]' — collection append
• 'requestBody' — scalar assignment

This eliminates the implicit $_nested arrays and reflection-heavy nesting logic from the classic pipeline.

Orphan detection

After hierarchy resolution, any remaining non-root attribute is an error — e.g. a Parameter on a class without a PathItem or Operation to absorb it. The Assembler throws
OpenApiException with source location context.

[DerManoMann]

Moved to zircote#2057 (base branch updated to master now that spec-dto is merged)