perf!: delta-aware BnbMetric via SelectionView/compute_view

[evanlinjin]

Summary

Makes the branch-and-bound metric evaluator delta-aware, eliminating the dominant cost in run_bnb. A flamegraph showed ~65% of run_bnb_lowest_fee time was spent in cs.selected_value() (47.5%) and cs.input_weight() (17.3%) — both O(|selected|) walks recomputed many times per branch via excess/is_target_met/drain.

Now BnB maintains a per-branch SelectionCache (running aggregates: value_sum, weight_sum, input_count, segwit_count, candidate_count). Inclusion expansions call cache.add(c) — O(1). Metrics receive a &SelectionView<'_> (a handle over &CoinSelector + Cow<SelectionCache>) whose read methods (selected_value, input_weight, excess, is_target_met, drain, …) are all O(1).

Outside the hot path, cs.compute_view() builds a fresh cache once on demand, collapsing the ~15 duplicated O(|selected|) read methods that previously lived on CoinSelector into a single entry point.

Benchmarks

Criterion median; delta-aware vs the bitset baseline, vs the pre-bitset master:

bench	master	bitset	delta-aware	total speedup
clone/4096	5.68 µs	58 ns	52 ns	109×
run_bnb_lowest_fee/20	470 µs	417 µs	158 µs	3.0×
run_bnb_lowest_fee/50	26.0 ms	10.3 ms	5.4 ms	4.8×
run_bnb_lowest_fee/100	73.2 ms	40.2 ms	14.2 ms	5.2×
run_bnb_lowest_fee/200	1.27 s	1.10 s	196 ms	6.5×

The gain is largest at large n, where selected_value/input_weight recomputation dominated.

Commits

1. perf!: delta-aware BnbMetric via SelectionView/compute_view — the core change. Introduces SelectionCache/SelectionView, moves the read methods off CoinSelector behind compute_view(), and makes the BnbMetric trait take &SelectionView. Adapted to the current trait API (target passed as a parameter; metric owns its change decision via drain(); Changeless<M>).
2. bench: extend pool sizes to wallet (1k) and exchange (10M) scale
3. perf: skip unselected scan in BnbIter via per-branch cursor — each Branch carries a cursor into candidate_order, so insert_new_branches jumps straight to the next undecided candidate instead of re-scanning selected/banned.
4. docs: fix unresolved rustdoc links + rustfmt reflow

API changes (breaking)

• New public SelectionView<'_>; SelectionCache is internal.
• BnbMetric::{score,bound,drain} now take &SelectionView<'_> instead of &CoinSelector<'_>.
• The ~15 read methods (selected_value, input_weight, excess, is_target_met, drain, fee, …) move from CoinSelector to SelectionView; reach them via cs.compute_view().

Testing

cargo fmt --check, cargo clippy --all-targets, RUSTDOCFLAGS="-D warnings" cargo doc --no-deps, and the full suite (33 lib+integration tests + 2 doctests) all pass. A proptest in src/selection_view.rs checks the cache invariant (cache.value_sum == cs.compute_view().selected_value()) against a from-scratch rebuild under random insert/deselect sequences.

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