mini-fanout: total_shares sums into u32 and can overflow

`distribute_v0` sums share weights into a `u32`:

```rust
let total_shares: u32 = share_indices
.iter()
.map(|&i| mini_fanout.shares[i].share.as_u32())
.sum();
```

`overflow-checks = true` is set for the release profile, so this panics rather than wraps once the
weights total more than `u32::MAX`. The weights are chosen by the fanout owner, so a fanout with
large enough `Share::Share` amounts cannot distribute at all: every attempt aborts.

The blast radius is that one fanout, and it is recoverable — the owner can lower the weights
through `update_mini_fanout_v0` — so this is not urgent. Widening the sum to `u128` is the fix;
`share.as_u128()` is already used a few lines below for the same value.

Worth a test that a fanout whose weights exceed `u32::MAX` still distributes, since nothing
currently exercises that range.

Raised during review of #1254 and deliberately kept out of it: that PR is a lint sweep, and this is
unrelated arithmetic in a program that moves tokens, so it deserves its own change and its own test
rather than riding an approved diff.

Locate the distribute_v0 entry point and inspect the total_shares calculation alongside the existing share.as_u128() use a few lines below. Add a regression test for weights exceeding u32::MAX and verify that the fanout still distributes; update_mini_fanout_v0 describes how weights can be lowered for recovery.

Written by the indexing model from the issue text.