fix(reader): reject a negative delta element count on its own

Self-review follow-up on #338. `rowCount > deltasLen - offset` was the
only thing standing between a negative `deltas_len` and the chunk loop,
and the subtraction wraps: `Long.MIN_VALUE - 1` is positive, so the
window check passed, the chunk range came out empty, and decode handed
back a zero-filled array of the requested length. A malformed file
answered instead of rejected — no raw exception, so ADR 0003 held, but
the file is still garbage and should say so.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Author: dfa1

reader/src/main/java/io/github/dfa1/vortex/reader/decode/DeltaEncodingDecoder.java

@@ -65,7 +65,11 @@ public Array decode(DecodeContext ctx) {
         // it — NegativeArraySizeException or OutOfMemoryError, neither a VortexException
         // (ADR 0003). Checked before any child decode, so a bogus length never drives an
         // allocation.
-        if (offset < 0 || rowCount > deltasLen - offset) {
+        // `deltasLen < 0` is checked on its own rather than left to the subtraction: a
+        // sufficiently negative one makes `deltasLen - offset` wrap positive, which passes a
+        // window check it should fail, and the chunk loop then simply does nothing and hands
+        // back a zero-filled array — a malformed file answered instead of rejected.
+        if (offset < 0 || deltasLen < 0 || rowCount > deltasLen - offset) {
             throw new VortexException(EncodingId.FASTLANES_DELTA,
                     "row window [" + offset + ", " + (offset + rowCount) + ") outside the "
                             + deltasLen + " delta element(s)");

reader/src/test/java/io/github/dfa1/vortex/reader/decode/DeltaEncodingDecoderTest.java

@@ -89,7 +89,8 @@ void decode_constantChildren_broadcastsAcrossChunk() {
             "1024, 0, 2000",     // more rows than the chunks reconstruct
             "1024, 900, 200",    // window starts inside the chunk but runs off its end
             "1024, -1, 4",       // negative start position
-            "-1024, 0, 4"        // negative element count
+            "-1024, 0, 4",       // negative element count
+            "-9223372036854775808, 1, 4"  // negative enough that `deltasLen - offset` wraps positive
     })
     void decode_windowOutsideDeltas_throws(long deltasLen, int offset, long rowCount) {
         // Given — metadata whose row window is not covered by `deltasLen` elements