Update instructions and fix build

CLAUDE.md

@@ -147,3 +147,16 @@ All mediaBox methods with clean signatures (no Postbox-protocol leaks, no comple
 **Facade-shape convention:** all of these take `EngineMediaResource.Id` or `EngineMediaResource` (never raw `MediaResourceId`/`MediaResource`). Return types either don't leak Postbox (`Void`, `String`, `String?`, `Signal<RangeSet<Int64>, NoError>`, `Signal<Float, NoError>`) or wrap via TelegramCore type (`Signal<EngineMediaResource.ResourceData, NoError>`).
 
 **Swift-stdlib-vs-third-party-module name collisions** (learned in wave 26): `RangeSet<Int64>` collides with Swift stdlib's `RangeSet` (iOS 18+ only). Fix: `import RangeSet` at the file top of any TelegramCore file that names `RangeSet` in a signature. `TelegramCore/BUILD` already depends on `//submodules/Utils/RangeSet:RangeSet`. Future facade additions in TelegramEngineResources.swift should re-check this if new signature types are introduced.
+
+## tgcalls Testbench
+
+This repo includes a tgcalls testbench (CLI tool, Go/Pion SFU, Docker build) layered on top of the iOS source. All testbench code, build instructions, and architecture docs live inside the tgcalls submodule:
+
+- `submodules/TgVoipWebrtc/tgcalls/CLAUDE.md` — top-level testbench overview, build/run commands
+- `submodules/TgVoipWebrtc/tgcalls/tools/cli/CLAUDE.md` — CLI test tool architecture
+- `submodules/TgVoipWebrtc/tgcalls/tools/go_sfu/CLAUDE.md` — Go SFU internals
+- `submodules/TgVoipWebrtc/CLAUDE.md` — tgcalls library internals + macOS/Linux build patches
+
+Build the test binary from this directory with:
+
+`./build-input/bazel-8.4.2 build //submodules/TgVoipWebrtc/tgcalls/tools/cli:tgcalls_cli`