refactor(writer): drop dead ptype arms in readPrimitiveElement

Like primitiveArrayLen, all callers (the global-dict counts/codes loops
and the post-exclusion isDictCandidate check) pass only dict-admitted
ptypes: I32/U32, I64/U64, F64. Collapse the unreachable I8/U8, I16/U16/F16,
F32 arms to default -> throw and add an F64 case to readPrimitiveElementCases.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: dfa1

writer/src/main/java/io/github/dfa1/vortex/writer/VortexWriter.java

@@ -1332,14 +1332,15 @@ static int primitiveArrayLen(Object data, PType ptype) {
         };
     }
 
+    /// Boxed element `i` of a global-dict column's chunk array. Only the dict-admitted carriers
+    /// ([#isDictCandidate]) — I32/U32, I64/U64, F64 — reach here; narrow-int and F16/F32 ptypes are
+    /// rejected upstream.
     static Object readPrimitiveElement(Object data, PType ptype, int i) {
         return switch (ptype) {
-            case I8, U8 -> ((byte[]) data)[i];
-            case I16, U16, F16 -> ((short[]) data)[i];
             case I32, U32 -> ((int[]) data)[i];
             case I64, U64 -> ((long[]) data)[i];
-            case F32 -> ((float[]) data)[i];
             case F64 -> ((double[]) data)[i];
+            default -> throw new IllegalStateException("ptype not admitted to the global dict: " + ptype);
         };
     }
 

writer/src/test/java/io/github/dfa1/vortex/writer/VortexWriterDictDecisionTest.java

@@ -120,9 +120,11 @@ void primitiveArrayLen_returnsActualLength(Object data, PType ptype, int expecte
     }
 
     static Stream<Arguments> readPrimitiveElementCases() {
+        // Only dict-admitted ptypes reach readPrimitiveElement: I32/U32, I64/U64, F64.
         return Stream.of(
                 arguments(new long[]{7, 8, 9}, PType.I64, 1, 8L),
-                arguments(new int[]{7, 8, 9}, PType.I32, 2, 9));
+                arguments(new int[]{7, 8, 9}, PType.I32, 2, 9),
+                arguments(new double[]{7.0, 8.0, 9.0}, PType.F64, 0, 7.0));
     }
 
     @ParameterizedTest