[umpireboulianne-design]

Summary

Implements a shared #![no_std] fixed-point (1e7) and basis-point (10_000) checked-math library in refract-math (math/) and migrates the four governance-adjacent contracts (backstop, bounty, grants, and treasury) onto it.

Key Changes

• Workspace & Coverage Integration (Cargo.toml, coverage-floor.toml):
  • Moves math from exclude into workspace members so cargo test --workspace and CI coverage gates execute refract-math's unit and proptest suites on every PR.
  • Adds [math] floor (lines = 95.00, branches = 90.00) to coverage-floor.toml.
• Fixed-Point (1e7), Basis-Point (10_000), and Checked-Math Primitives (math/src/lib.rs, math/src/tests.rs):
  • Adds SCALE_1E7: i128 = 10_000_000, MAX_BPS: u32 = 10_000, and Rounding::{Floor, Ceil}.
  • Adds mul_div(a, b, c, rounding) and mul_div_ceil(a, b, c) backed by 256-bit mul_wide / div_wide intermediates so intermediate products never overflow when the quotient fits in i128.
  • Adds apply_bps, apply_bps_floor, apply_bps_ceil, and ratio_bps with explicit validation (bps <= 10_000, 0 <= part <= whole).
  • Adds to_1e7 and from_1e7 scale conversion helpers plus checked_add, checked_sub, checked_mul, saturating_add, and saturating_sub.
  • Expands unit and proptest coverage for floor/ceil invariants, apply_bps_floor(x, bps) + apply_bps_ceil(x, 10_000 - bps) == x, and to_1e7 / from_1e7 round-trips across [0, i128::MAX].
• Contract Migrations:
  • backstop: Uses refract_math::{mul_div_floor, split_bps, saturating_add, saturating_sub, BPS_DENOM, SCALE_1E7} for share minting/redemption (calc_shares, quote_withdrawal), tranche_share_price, senior_coverage_ratio, and receive_premium.
  • bounty: Uses refract_math::{checked_add, checked_sub} for balance and reservation accounting in fund, award, create_payout, claim, sweep_expired, and available_balance_inner.
  • grants: Uses refract_math::{checked_add, checked_sub, saturating_add} for milestone sum checks, claimable balance accumulation, liability accounting, and rescue excess checks.
  • treasury: Uses refract_math::{apply_bps_floor, saturating_add, saturating_sub, MAX_BPS} in Reserve target calculation and inflow routing.

Closes #235