docs: explain why the matchmaking control plane is Go, fix stale status

Add a Go-vs-C#/Rust/C++ rationale for the matchmaking control plane to
TECH_STACK.md, and point at it from MATCHMAKING.md and README.md.

Also correct CLAUDE.md and README.md, which still described the backend
as unstarted/not built even though server/ has ~13k lines of Go across
matcher, allocator, api, store, security, supervisor and agones.

CLAUDE.md

@@ -6,7 +6,7 @@ Important rule: never create co-authored commits. Never mention Claude in commit
 
 ## Project overview
 
-Cosmic Clash is an open-source, physics-based "vehicle soccer" game (a spiritual successor to Rocket League) built in Godot 4.7, using space ships instead of cars. It is GDScript/Godot only today — the "C# backend" in README.md was never started, and the dedicated server is an export of this same Godot project. A **separate backend service is now planned** (not started) for casual/ranked matchmaking, which is a 1.0 launch blocker; see `docs/MATCHMAKING.md`. README.md's "MVP is local-only against bots" section is historical: server-authoritative online multiplayer, a headless dedicated server, Docker/CI verification, and an optional Steam transport are all implemented (Phases 1–6). See `multiplayer-next.md` for what actually remains.
+Cosmic Clash is an open-source, physics-based "vehicle soccer" game (a spiritual successor to Rocket League) built in Godot 4.7, using space ships instead of cars. The game and the dedicated server are GDScript/Godot only — the "C# backend" an early README described was never started, and the dedicated server is an export of this same Godot project. There is one component outside the Godot project: a **Go matchmaking control plane** in `server/` for casual/ranked queues, ranked ratings and Agones-based server allocation. It is partially implemented and a 1.0 launch blocker — see `docs/MATCHMAKING.md` for the design, `multiplayer-next.md` §0 and §7 for what remains (a real deployment cannot complete a match end to end today), and `docs/TECH_STACK.md` for why the control plane is Go rather than C#, Rust or C++. README.md's "MVP is local-only against bots" section is historical: server-authoritative online multiplayer, a headless dedicated server, Docker/CI verification, and an optional Steam transport are all implemented (Phases 1–6). See `multiplayer-next.md` for what actually remains.
 
 Because the gameplay concept (vehicle soccer) can't be copyrighted but specific expression can, all code/art/assets must be original — this is why the project uses Godot instead of Unreal/Unity and ships instead of cars. Keep this in mind when writing code or pulling in assets: don't port or closely mirror Rocket League's actual implementation.
 
@@ -19,8 +19,8 @@ The prose docs carry far more design rationale than the code comments, and sever
 - `SERVER.md` — dedicated-server build, config, systemd deploy, sizing.
 - `STEAM.md` — optional GodotSteam custom-build setup and the transport contract.
 - `FLIGHT_MANUAL.md` — the player-facing flight model.
-- `docs/MATCHMAKING.md` — casual/ranked queue design. Not implemented; a 1.0 launch blocker, and the reason a backend service now exists in the plan.
-- `docs/TECH_STACK.md` — what the project is built with and why.
+- `docs/MATCHMAKING.md` — casual/ranked queue design, and the locked constraints (Go/PostgreSQL/Redis/Agones) the `server/` module implements. Partially implemented; a 1.0 launch blocker, and the reason a backend service outside the Godot project exists at all.
+- `docs/TECH_STACK.md` — what the project is built with and why, including the Go-vs-C#/Rust/C++ rationale for the matchmaking control plane.
 - `TODO.md` — deferred non-multiplayer work (audio is the big one: there is none at all).
 
 ## Godot MCP server