refactor: extract basis-points range validation into assert_valid_bps helper (#704)

[christy-dev4]

Extract basis-points range validation into assert_valid_bps helper

Summary

Closes #704

Basis-point option fields (penalty_bps, tax_bps, insurance_premium_bps, …)
were validated inline at three call sites in lib.rs with
.expect("… must be ≤ 10000"). This PR extracts that check into a single shared
helper, assert_valid_bps, in contracts/split/src/validation.rs, matching the
existing assert_bps_sum / assert_bps_total guards, and routes all three call
sites through it.

This also fixes two pre-existing compile errors on main:

• error[E0428]: the name 'validation' is defined multiple times — lib.rs
  declared mod validation; twice (lines 58 and 74). The duplicate declaration
  has been removed.
• error[E0432]: unresolved import 'validation::assert_valid_bps' — the helper
  was referenced (line 77 and the three call sites) but never defined. It now
  exists.

What changed

• validation.rs — added assert_valid_bps(bps: u32) -> Result<(), ContractError>
  that returns Err(ContractError::InvalidRatio) when bps > BASIS_POINTS_TOTAL
  (10_000). u32 is already non-negative, so only the upper bound is checked.
• lib.rs — removed the duplicate mod validation; declaration. The three
  call sites already read assert_valid_bps(penalty_bps).expect(…) etc., so they
  now resolve correctly.

Behavior

No behavioral change for valid inputs. Out-of-range values still panic at the
call site via the existing .expect(...) (preserving the prior fail-fast
semantics); the validation logic is now centralized and reusable.

Verification

Resolves the E0428 (duplicate mod) and E0432 (assert_valid_bps unresolved)
errors. Note: cargo check on main still reports additional, unrelated
pre-existing errors from other unimplemented helpers/variants (e.g.
calc_platform_fee, TooFewRecipients, Map::has, Persistent::bump) that are
tracked by their own issues; this PR is scoped to #704 and does not attempt to
fix those.

closes #704

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