refactor: make Encoding.accepts abstract, fix silent opt-outs

Default return false hid two bugs: SequenceEncoding silently opted out
of the compressor despite having a working Primitive encoder; stubs
(Patched, Variant) and decode-only encodings (Masked, Pco) now declare
intent explicitly rather than inheriting a silent no-op.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: dfa1

core/src/main/java/io/github/dfa1/vortex/encoding/Encoding.java

@@ -17,13 +17,11 @@ public interface Encoding {
 	/// @return decoded array
 	Array decode(DecodeContext ctx);
 
-	/// Returns {@code true} if this encoding can encode the given dtype.
+	/// Returns `true` if this encoding can encode the given dtype.
 	///
 	/// @param dtype the dtype to test
-	/// @return {@code true} if this encoding accepts {@code dtype}
-	default boolean accepts(DType dtype) {
-		return false;
-	}
+	/// @return `true` if this encoding accepts `dtype`
+	boolean accepts(DType dtype);
 
 	/// Encodes {@code data} to bytes using the provided arena for output buffer allocation.
 	///

core/src/main/java/io/github/dfa1/vortex/encoding/MaskedEncoding.java

@@ -26,6 +26,11 @@ public EncodingId encodingId() {
 		return EncodingId.VORTEX_MASKED;
 	}
 
+	@Override
+	public boolean accepts(DType dtype) {
+		return false;
+	}
+
 	@Override
 	public EncodeResult encode(DType dtype, Object data, EncodeContext ctx) {
 		throw new VortexException(EncodingId.VORTEX_MASKED, "encode not yet implemented");

core/src/main/java/io/github/dfa1/vortex/encoding/PatchedEncoding.java

@@ -16,6 +16,11 @@ public EncodingId encodingId() {
 		return EncodingId.VORTEX_PATCHED;
 	}
 
+	@Override
+	public boolean accepts(DType dtype) {
+		return false;
+	}
+
 	@Override
 	public EncodeResult encode(DType dtype, Object data, EncodeContext ctx) {
 		throw new VortexException(EncodingId.VORTEX_PATCHED, "not yet implemented");

core/src/main/java/io/github/dfa1/vortex/encoding/PcoEncoding.java

@@ -67,6 +67,11 @@ public EncodingId encodingId() {
         return EncodingId.VORTEX_PCO;
     }
 
+    @Override
+    public boolean accepts(DType dtype) {
+        return false;
+    }
+
     @Override
     public EncodeResult encode(DType dtype, Object data, EncodeContext ctx) {
         return Encoder.encode(dtype, data, ctx);

core/src/main/java/io/github/dfa1/vortex/encoding/SequenceEncoding.java

@@ -37,6 +37,11 @@ public EncodingId encodingId() {
 		return EncodingId.VORTEX_SEQUENCE;
 	}
 
+	@Override
+	public boolean accepts(DType dtype) {
+		return dtype instanceof DType.Primitive;
+	}
+
 	@Override
 	public EncodeResult encode(DType dtype, Object data, EncodeContext ctx) {
 		return Encoder.encode(dtype, data, ctx);

core/src/main/java/io/github/dfa1/vortex/encoding/VariantEncoding.java

@@ -16,6 +16,11 @@ public EncodingId encodingId() {
 		return EncodingId.VORTEX_VARIANT;
 	}
 
+	@Override
+	public boolean accepts(DType dtype) {
+		return false;
+	}
+
 	@Override
 	public EncodeResult encode(DType dtype, Object data, EncodeContext ctx) {
 		throw new VortexException(EncodingId.VORTEX_VARIANT, "not yet implemented");