[evanlinjin]

Moves the crate's fee math from f32 to f64. With f32, rounding at ordinary amounts makes LowestFee miss the cheapest selection:

• LowestFee::bound cast selected_value and target.value() to f32 separately before subtracting them. At ~1 BTC, f32's spacing is 8 sats, so the bound could come out several sats above the true fee, and BnB pruned the cheapest selection.
• LowestFee's score cast the fee to f32, so fees of 2^24 sats or more that differ by a few sats scored the same (LowestFee score loses precision for fees above 2^24 sats due to f32 conversion #82).

Patching only bound would leave every other f32 path rounding, so the whole float layer changes. The next release breaks the API anyway:

• FeeRate wraps an Ordf64. Its constructors and as_sat_vb/spwu take and return f64.
• Candidate::effective_value, value_pwu, effective_value_pwu, implied_fee and fee_per_value, CoinSelector::waste and DrainWeights::waste return f64.
• BnbMetric::score/bound, run_bnb and bnb_solutions use Ordf64.
• FloatExt::ceil goes through i64 rather than i32, so it is correct for f64.
• LowestFee::bound subtracts the values as integers and needs no helper or final cast.

Tests: bound_is_admissible_at_large_values calls bound on a 1 BTC + 99 sat input (on master the bound is 104 while selecting both costs 101). score_tells_apart_fees_above_2_pow_24 checks that fees of 2^25 and 2^25 + 1 score differently. The round-count asserts in tests/bnb.rs are unchanged.

Fixes #82

Replaces #87. Differences from it:

• Converts the crate to f64 rather than adding a private f64 copy of Candidate::effective_value and casting the bound back to f32, which left the score and every other caller in f32.
• Based on current master: keeps the .max(0.0) clamp from fix: clamp ideal_fee to 0.0 in LowestFee::bound to prevent f32 rounding panic #71 where fix: compute LowestFee::bound in f64 #87 kept the assert! that fix: clamp ideal_fee to 0.0 in LowestFee::bound to prevent f32 rounding panic #71 removed.
• Uses deterministic unit repros instead of a 158-line brute-force file (7 × 20,000 cases, about 30s, not gated with #[cfg(not(debug_assertions))]).