Phase 5 — Package the core as a reusable lib

[ozgesolidkey]

Part of #14. Ref: docs/ARCHITECTURE_V2.md §7.

Ship L0–L2 (Host + Command Bus + Document + Editor/Viewer core + Decorations) as a reusable package consumed by LOGAN (logs) AND a nexum code-view surface (and a future web LOGAN).

Tasks

• Extract L0–L2 into an internal package with a clean public API.
• LOGAN consumes it as the first client (logs).
• Spike: a minimal nexum code-view surface as a second client (syntax highlight + git decorations on top).
• Spike: WebHost + serve L2–L4 as a browser app.

Done when: the core has ≥2 consumers and no LOGAN-specific code leaks into it.