Phase 2 — Editor/Viewer core + Decoration API

[ozgesolidkey]

Part of #14. Ref: docs/ARCHITECTURE_V2.md L2 (the reusable heart).

Extract the virtualized text surface as a standalone module that renders any Document. Introduce one Decoration model (line/range + style + gutter + minimap marker + click action).

Tasks

• Extract viewport rendering / scroll / minimap / selection / gutter / word-wrap / zoom into a viewer-core module.
• Define Decoration type + registry.
• Migrate highlights, annotations, agent findings, bookmarks, search hits onto Decorations (one render path).
• (Perf) compute heavy decoration sets off the main thread (worker).

Done when: viewer-core knows nothing about logs/code; all markers are Decorations.