Consolidate FigmaTransformer result finalization

[chubes4]

Parent: #1076

Problem

transformScenegraph(), transformResponsivePage(), and transformNormalizedScenegraphPage() separately attach mismatch reports, merge diagnostics, build parity data, construct source reports, calculate overlapping metrics, and create FigmaTransformResult envelopes. This makes output-contract changes easy to apply inconsistently.

Scope

• Introduce one result-finalization path for normalized artifacts.
• Keep normalization and page-selection differences in their existing branch-specific code.
• Centralize mismatch report attachment, diagnostics, parity, source-report construction, and common metrics.

Acceptance criteria

• composer test passes.
• Public result schemas and status behavior remain unchanged.
• Single-page, responsive, and pre-normalized page paths retain their current metrics and source-report fields.
• Layout and render-style mismatch evidence remains attached identically.
• No backward-compatibility wrapper or duplicate finalizer remains.

AI assistance

OpenAI gpt-5.6-sol via OpenCode identified the duplicated finalization paths and drafted this issue. Chris Huber is responsible for the scope and final changes.