Phase 4 — Document model

[ozgesolidkey]

Part of #14. Ref: docs/ARCHITECTURE_V2.md L1.

Formalize Document (line-addressable text + metadata: columns, level, timestamp, language) around the existing sourceAdapter. Features become Document-based, not 'log-line'-based.

Tasks

• Define Document interface (lineCount, getLines, search index, metadata) built from SourceAdapter output.
• Back it by fileHandler (line-offset index) server-side / chunked fetch on web.
• Migrate features off implicit 'log line' assumptions onto Document.
• Prove code/diff are 'just another Document type'.

Done when: a code file opens in the same viewer with no log-specific assumptions.