Support DeepSeek V4 Flash Vision Experimental

docs/issue100-vision/metadata.log

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+   Compiling ds4-server v0.1.0 (/Users/gb/Projects/DS4Server)
+    Finished `test` profile [unoptimized + debuginfo] target(s) in 4.00s
+warning: the following packages contain code that will be rejected by a future version of Rust: block v0.1.6
+note: to see what the problems were, use the option `--future-incompat-report`, or run `cargo report future-incompatibilities --id 1`
+     Running unittests src/main.rs (target/debug/deps/ds4_server-d15236c42a8179f4)
+
+running 1 test
+test engine::validation::tests::issue100_checkpoint_metadata_is_strict ... ok
+
+test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 568 filtered out; finished in 0.01s
+
+     Running unittests tools/qwen38-artifacts.rs (target/debug/deps/qwen38_artifacts-3a34dfcd9c4f3de8)
+
+running 0 tests
+
+test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured; 4 filtered out; finished in 0.00s
+
+     Running unittests tools/test-supervisor.rs (target/debug/deps/test_supervisor-978addc9e5e8dbda)
+
+running 0 tests
+
+test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured; 4 filtered out; finished in 0.00s