perf: hot-path quick wins — hoist getSimpleName, batch scratch buffer + BitSet, drop KeyOrdered wrapper

[eschizoid]

Summary

Three small, surgical allocation cuts on the per-record hot path. No behavior change; existing tests pass unmodified.

1. TypedPipelineBuilder.deserialize — hoist getSimpleName()

format.getClass().getSimpleName() was recomputed on every deserialize() call, but the value is only used inside the catch arm (diagnostic message wrap). Moved it to a final capture in build() so it's computed exactly once per built pipeline.

Allocation eliminated: one String plus a class-hierarchy walk per record on the success path.

2. BatchPipelineWrapper.flushLocked — scratch buffer reuse + BitSet

• Replaced List.copyOf(buffer) + separate ArrayList<T> for values with one pre-sized ArrayList<>(buffer) snapshot and one pre-sized values list. The buffer field is now reused (ArrayList.clear() keeps the backing array), so steady-state flushes don't reallocate the buffer.
• Replaced HashSet<Integer>(size * 2) for succeeded with BitSet(size) — constant-time set / get on primitive int indexes, zero Integer boxing on the walks.
• Replaced ArrayList<Integer> missing with a count-first pass; only allocate the list on the (rare) coverage-violation path. Common case is allocation-free for the missing-index tracking.

Allocations eliminated per flush: the immutable List.copyOf array, the HashSet plus its boxed Integer entries, the always-allocated missing list, and the buffer's backing array on every steady-state flush.

3. KeyOrderedDispatcher.dispatch — drop the wrapper Runnable

The dispatcher used to wrap (processTask, onComplete) in a per-dispatch lambda that also captured pending and key. Replaced with a small QueuedTask record holding only (processTask, onComplete). drain() now handles pending.decrement + onComplete directly in its own try/finally, preserving the same throw-safety + drain-loop-survives-any-Throwable semantics as the old wrapper.

The dispatcher's public dispatch(record, processTask, onComplete) signature is unchanged — all existing dispatcher tests pass without modification.

Allocation eliminated: the wrapper Runnable (captures 4 things) becomes a 2-field record. Lower object header + capture overhead per dispatch.

Deferred for JMH-baselined PRs

Bigger wins surfaced by the same audit are deliberately not in this PR — they need before/after benchmark numbers, not just "looks faster":

• OffsetManager: cache TopicPartition instances per (topic, partition) instead of allocating one per record's markOffsetProcessed / trackOffset.
• MessageFormat SPI: add deserialize(byte[], int, int) so the skipBytes(n) path can skip the System.arraycopy on every record.

Test plan

• ./gradlew :lib:kpipe-core:test — green (change 1 covered)
• ./gradlew :lib:kpipe-consumer:test — green twice, 263 unit tests pass on both runs. Two @Testcontainers integration tests (ExternalOffsetIntegrationTest, KPipeProducerIntegrationTest) fail with DockerClientProviderStrategy errors — pre-existing failure on main in any environment without a running Docker daemon, unrelated to these changes.
• All KeyOrderedDispatcherTest cases pass without modification (signature preserved).
• All BatchPipelineWrapper*Test cases pass — coverage contract, concurrency, and out-of-range index handling all green.

[eschizoid]

@copilot please review

[codecov]

Codecov Report

❌ Patch coverage is 88.46154% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 77.80%. Comparing base (4ccac98) to head (6f618c5).
⚠️ Report is 5 commits behind head on main.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
...eschizoid/kpipe/consumer/BatchPipelineWrapper.java	80.00%	0 Missing and 3 partials ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #203      +/-   ##
============================================
- Coverage     77.82%   77.80%   -0.02%     
+ Complexity      732      724       -8     
============================================
  Files            68       66       -2     
  Lines          2800     2798       -2     
  Branches        346      351       +5     
============================================
- Hits           2179     2177       -2     
+ Misses          462      461       -1     
- Partials        159      160       +1     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.