feat: tgcalls CLI test tool with group SFU, video, and adaptation

Squashed buildout of the tgcalls testbench:

- CLI test tool with --mode p2p/reflector/group/group-churn,
  cross-version interop (--version, --version2), and quiet/summary output
- Linux toolchain + Docker multi-stage build, AWS Fargate mass test harness,
  local parallel mass test harness with signaling loss simulation
- SCTP writable gate, retransmission timer tuning, role-based handshake
- InstanceV2CompatImpl (PeerConnection backend with V2Impl signaling) and
  SignalingTranslator for v14.0.0 interop
- In-process Go/Pion SFU (ICE+DTLS+SRTP+SCTP per participant) with audio
  RTP forwarding, ActiveAudio/VideoSsrcs data channel broadcast, RTCP
  feedback path, and CGo c-archive integration
- GroupInstanceReferenceImpl (PeerConnection group-call) and mixed-impl
  group mode (--reference-participants), with SDP munging for simulcast
- H264 simulcast group video (FakeVideoTrackSource pattern generator,
  FakeVideoSink frame counting, --video flag, two-pass channel setup,
  reactive video setup from ActiveVideoSsrcs)
- Group churn stress mode (--mode group-churn, --churn-cycles)
- SFU stream-quality adaptation: BandwidthEstimator, LayerSelector
  state machine, RtxRingBuffer, simulcast SSRC rewrite
- Transport-cc feedback generation, NetworkSimulator (delay/jitter/loss/
  token-bucket bandwidth), --network-scenario step-down-up
- CLAUDE.md updates throughout

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.bazelrc

@@ -1,32 +1,34 @@
 build --action_env=ZERO_AR_DATE=1
 
-build --apple_platform_type=ios
 build --enable_platform_specific_config
-build --apple_crosstool_top=@local_config_apple_cc//:toolchain
-build --crosstool_top=@local_config_apple_cc//:toolchain
-build --host_crosstool_top=@local_config_apple_cc//:toolchain
 
-build --per_file_copt=".*\.m$","@-fno-objc-msgsend-selector-stubs"
-build --per_file_copt=".*\.mm$","@-fno-objc-msgsend-selector-stubs"
-
-build --features=debug_prefix_map_pwd_is_dot
-build --features=swift.cacheable_swiftmodules
-build --features=swift.debug_prefix_map
-build --features=swift.enable_vfsoverlays
+# macOS-specific settings (auto-applied on macOS via --enable_platform_specific_config)
+build:macos --apple_platform_type=ios
+build:macos --apple_crosstool_top=@local_config_apple_cc//:toolchain
+build:macos --crosstool_top=@local_config_apple_cc//:toolchain
+build:macos --host_crosstool_top=@local_config_apple_cc//:toolchain
+build:macos --per_file_copt=".*\.m$","@-fno-objc-msgsend-selector-stubs"
+build:macos --per_file_copt=".*\.mm$","@-fno-objc-msgsend-selector-stubs"
+build:macos --features=debug_prefix_map_pwd_is_dot
+build:macos --features=swift.cacheable_swiftmodules
+build:macos --features=swift.debug_prefix_map
+build:macos --features=swift.enable_vfsoverlays
 build:dbg --features=swift.emit_swiftsourceinfo
 
+# Linux-specific settings (auto-applied on Linux via --enable_platform_specific_config)
+build:linux --action_env=CC
+build:linux --action_env=CXX
+
 build --strategy=Genrule=standalone
 build --spawn_strategy=standalone
 
-build --strategy=SwiftCompile=worker
+build:macos --strategy=SwiftCompile=worker
 
-#common --registry=https://raw.githubusercontent.com/bazelbuild/bazel-central-registry/main/
-
-# SourceKit BSP: Swift indexing features
-common --features=swift.index_while_building
-common --features=swift.use_global_index_store
-common --features=swift.use_global_module_cache
-common --features=oso_prefix_is_pwd
+# SourceKit BSP: Swift indexing features (macOS only)
+common:macos --features=swift.index_while_building
+common:macos --features=swift.use_global_index_store
+common:macos --features=swift.use_global_module_cache
+common:macos --features=oso_prefix_is_pwd
 
 # SourceKit BSP: Index build config (used for background indexing)
 common:index_build --experimental_convenience_symlinks=ignore