refactor(writer): drop dead ptype arms in primitiveArrayLen

All three callers (the global-dict writer's two chunk loops and the
isDictCandidate check, which runs after its narrow-int/F16/F32 early
return) pass only dict-admitted ptypes: I32/U32, I64/U64, F64. The
I8/U8, I16/U16/F16, and F32 arms were unreachable dead code. Collapse
to default -> throw and retarget the unit test's artificial I8 case to
F64.

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

Author: dfa1

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

@@ -1321,14 +1321,14 @@ static boolean isDictCandidate(PType ptype, Object data) {
         return seen.size() * 2 < n;
     }
 
+    /// Length 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 int primitiveArrayLen(Object data, PType ptype) {
         return switch (ptype) {
-            case I8, U8 -> ((byte[]) data).length;
-            case I16, U16, F16 -> ((short[]) data).length;
             case I32, U32 -> ((int[]) data).length;
             case I64, U64 -> ((long[]) data).length;
-            case F32 -> ((float[]) data).length;
             case F64 -> ((double[]) data).length;
+            default -> throw new IllegalStateException("ptype not admitted to the global dict: " + ptype);
         };
     }
 

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

@@ -100,10 +100,11 @@ void codePTypeForSize_picksNarrowestUnsignedCarrier(int dictSize, PType expected
     // ── primitiveArrayLen / readPrimitiveElement ─────────────────────────────────
 
     static Stream<Arguments> primitiveArrayLenCases() {
+        // Only dict-admitted ptypes reach primitiveArrayLen: I32/U32 (int[]), I64/U64 (long[]), F64.
         return Stream.of(
                 arguments(new long[]{1, 2, 3}, PType.I64, 3),
                 arguments(new int[]{1, 2}, PType.I32, 2),
-                arguments(new byte[]{1, 2, 3, 4}, PType.I8, 4));
+                arguments(new double[]{1.0, 2.0, 3.0, 4.0}, PType.F64, 4));
     }
 
     @ParameterizedTest