feat: add adaptive capacity scaling to ReorderBuffer

[oferchen]

Summary

• Introduce AdaptiveCapacityPolicy (sibling module concurrent_delta::adaptive) and compose it into ReorderBuffer via a new ReorderBuffer::with_adaptive_policy constructor. Default ReorderBuffer::new(capacity) behaviour is unchanged.
• Buffer grows by growth_factor (capped at max) when insert() finds utilization >= 80% AND the gap window > current capacity / 2. The grow path also kicks in pre-insert when an incoming sequence would otherwise be rejected, up to max.
• Buffer shrinks back toward min when the moving average of the last sample_window insert utilization samples falls below 25%, never evicting buffered items.
• ReorderStats { grow_events, shrink_events, capacity } is exposed through ReorderBuffer::stats().

Test plan

• cargo fmt --all (clean against changed files).
• CI: fmt+clippy, nextest (stable), Windows (stable), macOS (stable), Linux musl (stable).
• New unit tests:
  • adaptive_tests::fixed_capacity_default_unchanged - default constructor is unaffected.
  • adaptive_tests::adaptive_buffer_starts_at_min_capacity
  • adaptive_tests::grows_under_load
  • adaptive_tests::never_exceeds_max
  • adaptive_tests::shrinks_when_idle
  • adaptive_tests::never_drops_below_min
  • adaptive_tests::stats_track_both_events
  • adaptive_tests::ordering_preserved_through_resize
• Module-level tests in adaptive.rs cover policy bound validation, grow/shrink predicates, and rolling-window semantics.
• All pre-existing ReorderBuffer tests retained and continue to pass under the unchanged fixed-capacity default path.