perf: hot-path optimizations for enqueue!, poll!, and ack!

[Copilot]

Implements items 1–7 from the performance improvement plan targeting the enqueue/poll/ack hot paths.

Changes

• Branchless frame alignment — replace if (zero? r) with (bit-and (+ total frame-align-mask) (bit-not frame-align-mask))
• Binary segment search — find-segment-for-offset uses binary search instead of linear scan over the sorted segments vector
• Cached current segment — Queue gains a ^:volatile-mutable ^Segment current-seg field, eliminating (peek @segments) atom deref + vector peek on every enqueue!/poll!
• ByteBuffer.slice(index, length) — try-read-one! uses the 2-arg slice (Java 13+), dropping the .duplicate + position/limit dance (one fewer allocation per message)
• Fused lrbFrontier — single lrbFrontier(read-head) interface method replaces lrb-empty? + lrb-first two-dispatch sequence in ack!
• Volatile cursor-dirty — ^:volatile-mutable ^boolean cursor-dirty replaces the atom (AtomicReference CAS) on every async ack
• Segment pre-creation — at 75% segment fill, pre-creates the next segment file + mmap using a temp path; roll-segment! does an atomic rename and wraps the existing resources, avoiding file allocation/mmap latency on the roll-triggering write

Pre-creation design

enqueue! at 75% fill → precreate-next-segment!
  creates "seg-precreated.k7.tmp", opens FileChannel, mmaps

enqueue! triggers roll → roll-segment!
  Files/move(tmp → seg-{offset}.k7, ATOMIC_MOVE)
  wraps existing channel/mmap in new Segment with correct base-offset

close-queue! with unused pre-created segment
  close-segment! + Files/deleteIfExists

All 30 existing tests pass unchanged.