Add design spec for tgcalls CLI UDP reflector mode

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docs/superpowers/specs/2026-04-09-reflector-support-design.md

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+# tgcalls CLI: UDP Reflector Mode
+
+## Overview
+
+Add a `--mode reflector` option to the tgcalls CLI test tool, routing both call instances through a real Telegram UDP reflector instead of direct P2P loopback.
+
+## CLI Interface
+
+- `--mode p2p` — current behavior (direct P2P loopback, no servers)
+- `--mode reflector` — route through a real Telegram UDP reflector
+- `--mode` is **required**. Exit with usage error if missing.
+- `--reflector host:port` — specifies the reflector address. **Required** when `--mode reflector`. Error if missing in reflector mode or if provided with `--mode p2p`.
+- `--duration` and `--quiet` are unchanged.
+
+## Reflector Configuration
+
+When `--mode reflector`:
+
+### Peer Tag Generation
+
+Generate 16 random bytes. Copy to make two tags:
+- Caller tag: byte 0 = `0x00`
+- Callee tag: byte 0 = `0x01`
+
+### RtcServer Setup
+
+Each instance gets one `RtcServer` entry with its respective peer tag:
+
+| Field      | Value                                          |
+|------------|------------------------------------------------|
+| `id`       | `1`                                            |
+| `host`     | from `--reflector` argument                    |
+| `port`     | from `--reflector` argument                    |
+| `login`    | `"reflector"`                                  |
+| `password` | hex-encoded 16-byte peer tag (differs by side) |
+| `isTurn`   | `true`                                         |
+| `isTcp`    | `false`                                        |
+
+### Descriptor Changes
+
+- `config.enableP2P = false`
+- `rtcServers` populated with the single reflector server
+- No `customParameters` changes (standalone reflector mode is not used)
+
+### P2P Mode
+
+Unchanged from current behavior: `enableP2P = true`, empty `rtcServers`.
+
+## Summary Output
+
+Add a mode line to the call summary:
+
+```
+Mode:              reflector (91.108.13.2:596)
+```
+
+or:
+
+```
+Mode:              p2p
+```
+
+No other output changes. The existing audio validation (440Hz sine, non-silence detection) remains the success criterion for both modes.