[Elvand-Lie]

This Pull request:

Changes or fixes:

Width-weighted histogram projections can retain statistics from the unweighted source histogram.

double y[] = {0., 1., 4.};
TH2D h("h", "", 2, 0., 2., 2, y);
h.Fill(0.5, 0.5); h.Fill(1.5, 2.);
auto p = h.ProjectionX("px", 1, 2, "width");
// Bin contents: 1, 3. Before: mean 1. After: mean 1.25.

The original statistics cannot be reused when integration includes bin widths. Recalculate the statistics for width-weighted TH2 projections and for both TH3 projection paths, even when the weighted total equals the original total. The recalculated moments use projected bin centers, since width-dependent event-level moments are not retained. Regression tests cover both TH2 projection axes and one- and two-dimensional TH3 projections.

Both regression tests fail on the base and pass with the fix. The full test_projections executable passes (5 tests) on Linux. The full ROOT test suite and other platforms were not run.

Checklist:

• tested changes locally
• updated the docs (if necessary)

Codex (gpt-6.1-terra) implemented the fix and the tests. I reviewed the full diff and the tests.