bench: add ChangelessWaste bound-version comparison

Adds a Criterion bench that runs four side-by-side bound implementations:
  V1: simple input_weight * rate_diff
  V2: V1 + resize trick for target-not-met, rate_diff >= 0
  V3: V2 + slurp tightening for target-met-with-change, rate_diff >= 0
  V4: V2 + LP-relaxed knapsack UB for rate_diff < 0 (matches current src)

Prints a round-count comparison table at the start of the run (helps see
relative bound tightness independent of constant-time overheads) and then
times each version on shared synthetic pools. Each Vn is a self-contained
metric type implementing BnbMetric so future tightenings can be added and
compared without disturbing the production code.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: evanlinjin

Cargo.toml

@@ -29,3 +29,7 @@ criterion = "0.5"
 [[bench]]
 name = "coin_selector"
 harness = false
+
+[[bench]]
+name = "changeless_waste_bounds"
+harness = false