fix(leasing): floor per-contributor dividend share so beneficiary gets the leftover

[boskodev790]

## Summary

`distribute_leased_network_dividends` rounded each contributor's alpha
share UP with `.ceil()` before accumulating into `alpha_distributed`.
Summed across contributors the ceilings can exceed
`total_contributors_cut_alpha`, zeroing the beneficiary's `saturating_sub`
leftover on line ~327 even though `raised - deposit > 0` always
entitles them to some dust.

In the degenerate case where cumulative over-allocation exceeds the
lease coldkey's actual subnet stake, `transfer_stake_within_subnet`
fails mid-loop, the whole `with_storage_layer` block rolls back, and
dividends re-accumulate for the next interval — breaking the
distribute-at-each-interval invariant.

The top-level `lease.emissions_share.mul_ceil(owner_cut_alpha.to_u64())`
on line ~273 still rounds UP in favour of the contributors group as a
whole (deliberate, called out by the comment there). This PR only
changes the per-contributor rounding inside the distribution loop.

## Changes

- `pallets/subtensor/src/subnets/leasing.rs`: switch the per-contributor
  `.ceil()` on line 305 to `.floor()` so the sum of all per-contributor
  shares is always `<= total_contributors_cut_alpha`. Expand the
  accompanying comment to explain the invariant and why the higher-up
  `mul_ceil` is still correct.
- `pallets/subtensor/src/tests/leasing.rs`: update the two matching
  `.ceil()` expected-value computations in
  `test_distribute_lease_network_dividends_multiple_contributors_works`
  so the expected per-contributor alpha tracks the production rounding.
  The `distributed_alpha == bene + c1 + c2` assertion now holds
  unconditionally (previously only when U64F64 happened to round
  cleanly for the specific fixture shares).

## Testing

- [x] Existing test suite for leasing dividends unchanged in structure;
      the two `expected_contributorN_alpha` computations are updated
      in lock-step with production.
- [ ] `cargo check -p pallet-subtensor` / `cargo test -p pallet-subtensor --test leasing`
      — not run locally because the Substrate toolchain + native
      dependencies are not installed in my environment. Happy to run
      locally and paste output if the reviewer prefers.

## Related Issues

N/A — code review. No existing issue or PR addresses this rounding
asymmetry; verified via GitHub search for
`distribute_leased_network_dividends`, `SubnetLeaseShares + ceil`,
`alpha_for_contributor`.

## Checklist

- [x] Code follows project style guidelines (mirrors the existing
      `saturating_to_num::<u64>()` idiom in the same function)
- [x] Self-review completed
- [x] No runtime storage / public API change — only rounding semantics
      inside an existing hook