style(qwp): javadoc buildCrc32cTable; fix imprecise test msg (#64 m2)

Address the two cosmetic follow-ups from the #64 review:
- buildCrc32cTable now uses a /** */ javadoc, matching its sibling
  private helpers crc32cRecovery/detectTornTail (was a // comment).
- The payload-unbacked test asserted the frame-2 header "must end on
  the page boundary"; it actually ends 8 bytes below it (the asserted
  boundary-8 value, per the javadoc above). Reword to match.

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

Author: puzpuzpuz

core/src/main/java/io/questdb/client/cutlass/qwp/client/sf/cursor/MmapSegment.java

@@ -668,9 +668,12 @@ private static int crc32cRecovery(long addr, long len) {
         return ~crc;
     }
 
-    // Standard reflected CRC-32C byte table (polynomial 0x82F63B78), matching
-    // crc32c_table[0] in the native crc32c.c. Computed at class init to avoid
-    // 256 hand-transcribed literals; drives crc32cRecovery.
+    /**
+     * Standard reflected CRC-32C byte table (polynomial {@code 0x82F63B78}),
+     * matching {@code crc32c_table[0]} in the native {@code crc32c.c}. Computed
+     * at class init to avoid 256 hand-transcribed literals; drives
+     * {@link #crc32cRecovery}.
+     */
     private static int[] buildCrc32cTable() {
         int[] table = new int[256];
         for (int n = 0; n < 256; n++) {

core/src/test/java/io/questdb/client/test/cutlass/qwp/client/sf/cursor/MmapSegmentRecoveryFaultTest.java

@@ -162,7 +162,7 @@ public void testRecoverySurvivesPayloadReachingUnbackedPage() throws Exception {
             final int payloadLen1 = (int) (frame2Offset - MmapSegment.HEADER_SIZE - MmapSegment.FRAME_HEADER_SIZE);
 
             long used = writeSegment(path, 11L, new int[]{payloadLen1, payloadLen2});
-            assertEquals("frame 2's header must end on the page boundary", boundary - 8, frame2Offset + MmapSegment.FRAME_HEADER_SIZE);
+            assertEquals("frame 2's header must end 8 bytes below the page boundary", boundary - 8, frame2Offset + MmapSegment.FRAME_HEADER_SIZE);
             assertTrue("frame 2 payload must reach past the boundary", used > boundary);
             punchSparseTail(path, boundary);