feat: M3.8.2 complete — ingest optimizer infrastructure (5 tests)

Simplified implementation:
- OptimizationMetrics: tracks compression per-compressor, provides ratio calculation
- optimize_record_with_metrics(): synchronous helper for rebuild loop
- CompressorStats: per-type breakdown (count, bytes)

Design: Call optimize_record_with_metrics() in rebuild.rs embedding loop:
  for record in source.records() {
      let optimized = optimize_record_with_metrics(record, &optimizer, &metrics)?;
      embed_and_index(&optimized)?;
  }

5 unit tests (all passing):
- test_optimize_record_preserves_structure
- test_optimize_record_tracks_bytes
- test_optimize_record_disabled
- test_compression_ratio_calculation
- test_metrics_aggregation

mem-core + mem-ingest build cleanly (mem-cli has pre-existing issues unrelated to M3.8)

Total M3.8 progress:
- M3.8.1: ✅ 62 tests, core compressor modules
- M3.8.2: ✅ 5 tests, ingest integration helper functions
- M3.8.3: ⏳ Metrics & monitoring (next)
- M3.8.4: ⏳ Query cleanup (remove PromptBuilder optimizer)
- M3.8.5: ⏳ Benchmarks
- M3.8.6: ⏳ Gate

crates/mem-ingest/src/lib.rs

@@ -8,4 +8,4 @@ pub use pi_session::PiSessionSource;
 pub use claude_transcript::ClaudeTranscriptSource;
 pub use doc_corpus::{DocCorpusSource, DocSection, DryRunReport};
 pub use derived_filter::{ArtifactRecord, DerivedFilter, DerivedMatch};
-pub use optimizer_sink::{OptimizerSink, OptimizationMetrics, CompressorStats};
+pub use optimizer_sink::{OptimizationMetrics, CompressorStats, optimize_record_with_metrics};