docs(phase-2): add explanation pages (architecture, game-state, runtime)

Part of #224 — v2 docs rewrite (Diátaxis).

## Goal

Explanation pages: `architecture.md`, `game-state.md` (omniscience/fair-play), `runtime.md` (platforms + rendering) — each 1.5–3k words, code-cited, titled "About X".

## Tasks

- [x] Read `src/lua/core/*`, `utils/gamestate.lua`, `platforms/*`, ADRs 0001–0003; write with maintainer review
- [x] Cross-link to reference pages (never duplicate facts)
- [x] One question per page; concrete examples

## Dependencies

- Blocked by: #226 (Phase 1 — reference rewrite)
- Blocks: Phase 3 (how-to), Phase 5 (ship)

## Done when

- [ ] 3 pages reviewed by a maintainer
- [x] Code paths cited with GitHub links

Review the drafts for architecture.md, game-state.md, and runtime.md against src/lua/core/*, utils/gamestate.lua, platforms/*, and ADRs 0001–0003. Confirm that each page is titled “About X,” answers one question, uses concrete examples, cites code paths, and cross-links rather than duplicates the Phase 1 reference pages. Done means all three pages have maintainer review; check #226 first because this issue depends on it.

Written by the indexing model from the issue text.