docs(architecture): define greenfield Rust Oteryn client

[blakinio]

Purpose

Define one normative, agent-ready architecture for a greenfield Rust Oteryn client while keeping the current C++/Lua/OTUI client operational as a separately documented legacy/reference track.

Greenfield scope

• isolated oteryn-client/ product directory and nested agent rules
• data-oriented runtime, chunked world model and wgpu renderer boundary
• native Rust UI, input, audio, assets, diagnostics and launcher/updater boundaries
• static first-party feature crates with optional capability-limited WebAssembly extensions
• Canary-first protocol adapter and separate future Oteryn adapter over stable GameEvent/GameCommand contracts
• Oteryn Identity Authorization Code + PKCE and one-shot game-session handoff
• account, character, world and gameplay-channel selection
• Channel 1 -> Channel 2 changes through relog with a fresh ticket/session
• measurable performance/test/security requirements

Agent program

• mandatory foundation audit before Cargo/workspace bootstrap
• ten required audit outputs covering product, Canary, Identity/session, assets/licensing, performance, platform, fixtures, risks and decisions
• workstream/path ownership map
• copy-ready prompt for the first audit-only agent
• five accepted ADRs for the core architecture decisions

Legacy preservation

The existing client remains untouched and buildable. Its maintenance knowledge is preserved in:

• docs/architecture/LEGACY_OTCLIENT_ARCHITECTURE.md
• docs/agents/LEGACY_OTCLIENT_WORKSTREAMS.md

Legacy code is audit evidence only for the Rust track and is not a runtime dependency or architectural template.

Product constraint

A gameplay channel is a parallel instance of one world. It is selected during login or relog; it is not a network multiplexing stream. Seamless in-game channel transfer is out of scope.

Safety and review

• documentation/governance paths only
• no production Rust crates yet
• no C++/Lua runtime, workflow, binary or asset changes
• no secrets, proprietary assets or external-repository writes
• active PR fix(assets): select release archives by client version #37, fix(options): repair deterministic Phase 0 defects #36 and feat(ui): prototype Oteryn login shell #23 implementation ownership unchanged
• full changed-file/patch review identified and repaired an initial loss of legacy governance detail

Validation

• complete 31-file documentation/governance path review: passed
• architecture, route, terminology and legacy/Rust separation review: passed
• exact-head CI run 30222662629: passed
• Detect Build Scope, both Fast Checks, Lua Syntax and CI / Required: passed
• Windows build correctly skipped for documentation-only scope
• no open comments or requested changes

Task

OTC-20260726-rust-client-architecture

Ready for squash merge after the final mergeability, changed-file and required-check verification.