fix: MultiCell loses contents when a growing axis resizes storage

[NJManganelli]

Problem

Filling a MultiCell histogram that has a growing axis (e.g. StrCategory(growth=True))
silently zeroes everything filled before the axis grows. Any category that appears
after the first fill triggers a storage resize, and all previously accumulated cells
are lost. The only workaround today is to declare every category up front.

import boost_histogram as bh, numpy as np

def make(cats):
    return bh.Histogram(bh.axis.StrCategory(cats, growth=True),
                        bh.axis.Regular(3, 0, 3),
                        storage=bh.storage.MultiCell(2))

grown, pre = make([]), make(["a", "b", "c"])
for chan, xs, cells in [("a",[.5,1.5,2.5],[[1,10],[2,20],[3,30]]),
                        ("b",[.5,2.5],[[4,40],[5,50]]),
                        ("c",[1.5],[[6,60]])]:
    grown.fill(chan, np.asarray(xs), weight=np.asarray(cells, float))
    pre.fill(chan,  np.asarray(xs), weight=np.asarray(cells, float))

grown.view().sum()  # 13.0 + 130.0  -> wrong, most of it zeroed on resize
pre.view().sum()    # 28.0 + 280.0  -> correct

Cause

multi_cell_reference is a proxy reference (a span over a cell's elements). Its
element-wise assignment is provided only as a template operator=. A templated
assignment operator does not suppress the implicitly-declared copy-assignment
operator, so when both sides have the same type — multi_cell_reference = multi_cell_reference —
overload resolution prefers the compiler-generated copy-assignment, which just rebinds
the span instead of writing through to the referenced cells.

boost::histogram relocates cells with *new = *old in
detail::storage_grower::apply when a growing axis enlarges the storage. With the
rebinding assignment the relocation is a no-op, so every growth discards the existing
contents. (Confirmed by reproducing against pristine boost::histogram develop: the
cell-relocation offsets are computed correctly, but *ns = x never copies.)

Fix

Provide an explicit write-through copy-assignment operator for multi_cell_reference,
and default the copy constructor to keep proxy rebind-on-copy semantics. This is the
same contract every other boost::histogram storage reference already honours.

Adds a regression test that fills a growing MultiCell histogram category by category
and checks it matches the pre-declared-categories result. Full test suite passes; clean
under -Wall -Wextra and clang-format.

Follow-up to #1008 (MultiCell storage).

🤖 Investigated and drafted with Claude Code; reviewed by @NJManganelli.