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

[S1M0N38]

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

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

Dependencies

• Blocked by: docs(phase-1): rewrite reference pages (api, cli, configuration) #226 (Phase 1 — reference rewrite)
• Blocks: Phase 3 (how-to), Phase 5 (ship)

Done when

• 3 pages reviewed by a maintainer
• Code paths cited with GitHub links

[S1M0N38]

Phase 2 work is complete on branch dev-docs (commit 6ed3af4), still unable to push from this machine (read-only S1M0N38 access — see #226 comment).

Three pages added under docs/explanation/, each ~1.5k words, one question per page, code-cited with GitHub blob/dev links, cross-linked to the reference pages with no duplicated facts:

• About the BalatroBot architecture — the mod (Lua) + CLI (Python) split, Lovely/SMODS injection, boot sequence, the between-frames single-client HTTP server, the four-tier request lifecycle (protocol/schema/state/execution), the OpenRPC contract, debug endpoints, env-var config flow, pool/state file, a fresh-serve walkthrough, and how tests map onto the architecture.
• About game state — ante/round/blind lifecycle, the G.STATES machine and per-state method rules, conditional area presence, omniscience (API always returns hidden cards true values) vs fair play (the sort and transient revealed signals), GAME_OVER/paused detection, and a worked "is the small blind beatable" example.
• About runtime — local-process platforms (DYLD_INSERT_LIBRARIES / version.dll / Proton WINEDLLOVERRIDES+STEAM_COMPAT / LD_PRELOAD) vs the balatrobox container platform, instance lifecycle + health wait + no-restart design, the save-profile gate and runtime profiles (ADR 0001), render modes, and screenshot rendering/settling/hover (ADRs 0002, 0003).

Also wired the pages into the mkdocs nav (Reference/Explanation sections) and llmstxt, and enabled the navigation.sections theme feature. make quality green; strict mkdocs build passes with only the pre-existing llms-full.txt index warning.

The one open Done-when item is maintainer review — these need a human (ideally @S1M0N38 or the repo maintainer) to sanity-check the terminology and emphasis before Phase 3 links to them.