refactor: add overflow-safe CeilDiv helper

Introduce `CeilDiv()` for integral ceiling division without the typical `(dividend + divisor - 1) / divisor` overflow, asserting a non-zero divisor.

Replace existing ceiling-division expressions with `CeilDiv()` to centralize the preconditions.

Add unit tests covering return type deduction, max-value behavior, and divisor checks.

src/common/bloom.cpp

@@ -12,6 +12,7 @@
 #include <span.h>
 #include <streams.h>
 #include <util/fastrange.h>
+#include <util/overflow.h>
 
 #include <algorithm>
 #include <cmath>
@@ -166,7 +167,7 @@ CRollingBloomFilter::CRollingBloomFilter(const unsigned int nElements, const dou
      * restrict it to the range 1-50. */
     nHashFuncs = std::max(1, std::min((int)round(logFpRate / log(0.5)), 50));
     /* In this rolling bloom filter, we'll store between 2 and 3 generations of nElements / 2 entries. */
-    nEntriesPerGeneration = (nElements + 1) / 2;
+    nEntriesPerGeneration = CeilDiv(nElements, 2u);
     uint32_t nMaxElements = nEntriesPerGeneration * 3;
     /* The maximum fpRate = pow(1.0 - exp(-nHashFuncs * nMaxElements / nFilterBits), nHashFuncs)
      * =>          pow(fpRate, 1.0 / nHashFuncs) = 1.0 - exp(-nHashFuncs * nMaxElements / nFilterBits)
@@ -182,7 +183,7 @@ CRollingBloomFilter::CRollingBloomFilter(const unsigned int nElements, const dou
      * treated as set in generation 1, 2, or 3 respectively.
      * These bits are stored in separate integers: position P corresponds to bit
      * (P & 63) of the integers data[(P >> 6) * 2] and data[(P >> 6) * 2 + 1]. */
-    data.resize(((nFilterBits + 63) / 64) << 1);
+    data.resize(CeilDiv(nFilterBits, 64u) << 1);
     reset();
 }