feat: add SuperDeck layout diagnostics

[leoafarias]

Summary

• add non-intrusive debug overlays for slide sections, blocks, margins, and padding
• allow slide captures to opt into debug layout guides without sharing cache entries with normal captures
• make Markdown block spacing style-owned, with a zero renderer baseline and a 12px SuperDeck default
• update generated style code, capture timing coverage, layout tests, and affected goldens

Scope

This PR intentionally contains only reusable SuperDeck package changes. It excludes all Playground, Wizard, editor, planning, and session files. The single PDF-plugin test helper change only mirrors the updated SlideCaptureService.captureSlide signature.

Validation

• fvm dart run build_runner build (packages/superdeck)
• fvm flutter analyze (packages/superdeck)
• fvm flutter test test/src/styling/components/slide_test.dart test/src/capture/thumbnail_capture_timing_test.dart test/goldens/slide_goldens_test.dart (packages/superdeck)
• fvm flutter test test/src/pdf_controller_test.dart test/src/pdf_plugin_test.dart test/src/pdf_export_screen_test.dart (packages/plugins/pdf)