[tree] Fix long64 tree index losing precision on 64-bit long double platforms

The conditional `long64major ? GetLong64() : GetAndRangeCheck()` mixes
Long64_t and LongDouble_t, so the exact value was promoted through long
double regardless of branch. This rounded large values where long double
is 64-bit (macOS arm64, Windows), making
roottest-root-tree-index-indexl64 fail there while passing on Linux. Fix
this with an explicit `static_cast<Long64_t>(ret)`.

🤖 Done with the help of [Claude Code](https://claude.com/claude-code) (Claude Opus 4.8)

Author: guitargeek

tree/treeplayer/src/TTreeIndex.cxx

@@ -189,6 +189,7 @@ TTreeIndex::TTreeIndex(const TTree *T, const char *majorname, const char *minorn
          Error("TTreeIndex", "In tree entry %lld, Ndata in formula is zero for both '%s' and '%s'", i,
                fMajorName.Data(), fMinorName.Data());
       }
+
       auto GetAndRangeCheck = [this](bool isMajor, Long64_t entry) {
          LongDouble_t ret = (isMajor ? fMajorFormula : fMinorFormula)->EvalInstance<LongDouble_t>();
          // Check whether the value (vs significant bits) of ldRet can represent
@@ -206,11 +207,15 @@ TTreeIndex::TTreeIndex(const TTree *T, const char *majorname, const char *minorn
                     "In tree entry %lld, %s value %s=%Lf possibly out of range for internal `long double`", entry,
                     isMajor ? "major" : "minor", isMajor ? fMajorName.Data() : fMinorName.Data(), ret);
          }
-         return ret;
+         return static_cast<Long64_t>(ret); // static_cast necessary for use in ternary expression below
       };
       auto GetLong64 = [this](bool isMajor) {
          return (isMajor ? fMajorFormula : fMinorFormula)->EvalInstance<Long64_t>();
       };
+      // Note: in ternary expression ensure both branches have same return data type Long64_t.
+      // otherwise, with LongDouble_t, the expression is implicitly cast to their
+      // common type `long double` regardless of the branch taken. On platforms where
+      // `sizeof(long double) == sizeof(double)` (macOS arm64, Windows) this silently
       tmp_major[i] = long64major ? GetLong64(true) : GetAndRangeCheck(true, i);
       tmp_minor[i] = long64minor ? GetLong64(false) : GetAndRangeCheck(false, i);
    }