feat: M3.8.2 optimizer infrastructure — metrics collection + wrap_source helper

M3.8.2 Implementation (partial):
- OptimizerSink struct: holds optimizer + metrics
- OptimizationMetrics: tracks compression stats per-compressor
- wrap_source() function: wraps RecordSource with async optimization
- 4 unit tests for wrap_source

Note: wrap_source uses async .then() pattern. Full integration with rebuild.rs
pending in M3.8.2b (direct optimization in rebuild pipeline is simpler).

All projects build cleanly. Tests added but not yet run (require tokio integration).

Key achievement: Core infrastructure ready for ingest-time optimization.
Next: Wire into rebuild.rs rebuild loop for actual use.

crates/mem-ingest/src/lib.rs

@@ -2,8 +2,10 @@ pub mod pi_session;
 pub mod claude_transcript;
 pub mod doc_corpus;
 pub mod derived_filter;
+pub mod optimizer_sink;
 
 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};