Implement native VivoxTest validation (#95)

docs/vivox.md

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+# Native Vivox validation
+
+MetaCrate's Vivox support is a cross-platform Rust implementation of the SDK
+control protocol. `VivoxControlClient` connects to a caller-supplied TCP socket,
+writes bounded XML requests terminated by the Vivox three-newline delimiter,
+correlates responses by request ID and action, and queues asynchronous login,
+session, and participant events. It does not start or ship proprietary code.
+
+The client owns connector, account, and session state. Invalid ordering is
+rejected locally. `shutdown()` is idempotent and reverses active state in this
+order: sessions, account, connector, TCP writer. Response failures preserve the
+action and numeric return/status codes while passwords, handles, and URIs remain
+opaque. XML is escaped on output; input is capped at 1 MiB and rejects DTD or
+entity declarations.
+
+`vivox-test --fake-script FILE` launches a deterministic fake control service on
+IPv4 loopback. It validates the same wire requests used with a real daemon,
+scripts device responses and participant events, and waits for both client and
+server teardown. Live mode additionally logs into a grid, waits for the event
+queue, requests `ProvisionVoiceAccountRequest` and `ParcelVoiceInfoRequest`, and
+then exercises the control service. Live login requires both
+`--allow-live-login` and `--confirm-live-login LOGIN`; joining audio also requires
+`--allow-session-audio`.
+
+Use environment variables for live credentials so they are not copied into a
+shell history or process argument list:
+
+```sh
+GRID_FIRST_NAME=... GRID_LAST_NAME=... GRID_PASSWORD=... \
+  cargo run -p libremetaverse-programs --bin vivox-test -- \
+  --allow-live-login --confirm-live-login LOGIN
+```
+
+The Vivox daemon/SDK is proprietary and must be installed, configured, and
+started independently. MetaCrate never discovers or invokes it.