docs: add docs folder (architecture, developer, usage); update README; wire probe TTL cache + concurrency notes into docs

docs/developer/setup.md

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+# Developer Setup
+
+## Prerequisites
+- Rust toolchain (stable)
+- Node.js (LTS) for control/ui
+- Docker (optional) for local stack
+
+## Build
+```bash
+cargo build
+cd control/ui && npm ci && npm run build
+```
+
+## Workspace Verification
+```bash
+cargo fmt --check
+cargo clippy --workspace --all-targets -- -D warnings
+cargo test --workspace
+cd control/ui && npm ci && npm run lint && npm run typecheck && npm run test && npm run build
+```
+
+## Environment
+- Gateway: routing config using file or KV
+- Projection: PROJECTION_GRPC_ADDR
+- Runner: RUNNER_GRPC_ADDR
+- NATS: URLs via service-specific settings

docs/developer/testing.md

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+# Testing
+
+## Unit and Integration
+```bash
+cargo test --workspace
+```
+
+## Gated Tests (require external services)
+- Runner NATS:
+```bash
+RUNNER_TEST_NATS_URL=nats://127.0.0.1:4222 cargo test -p runner -- --ignored
+```
+- Projection NATS:
+```bash
+PROJECTION_TEST_NATS_URL=nats://127.0.0.1:4222 cargo test -p projection -- --ignored
+```
+- Docker-based gates:
+```bash
+cargo test -p gateway -- --ignored
+```
+
+## Control UI
+```bash
+cd control/ui
+npm ci
+npm run test
+```