feat: M3.8.3 complete — metrics & monitoring (7 tests)

MetricsCollector implementation:
- Per-project aggregation of OptimizationMetrics
- Structured logging via tracing (log_all_projects)
- Prometheus export format (prometheus_export)
- Per-compressor stat tracking

7 new tests (all passing):
- test_collector_merge_single_project
- test_collector_merge_multiple_projects
- test_collector_merge_aggregates
- test_collector_nonexistent_project
- test_collector_per_compressor_stats
- test_prometheus_export_format
- test_prometheus_compression_ratio

Ready to integrate into rebuild.rs:
  let collector = MetricsCollector::new();
  ...
  collector.merge_project(project_id, metrics);
  collector.log_all_projects();

Total M3.8 progress:
- M3.8.1: ✅ 62 tests (core compressors)
- M3.8.2: ✅ 5 tests (ingest helpers)
- M3.8.3: ✅ 7 tests (metrics & monitoring)
- M3.8.4: ✅ IMPLICIT (no query compression needed)
- M3.8.5: ⏳ Benchmarks
- M3.8.6: ⏳ Gate

79 tests passing total (62+5+7+5 from optimizer_sink)

crates/mem-ingest/src/lib.rs

@@ -3,9 +3,11 @@ pub mod claude_transcript;
 pub mod doc_corpus;
 pub mod derived_filter;
 pub mod optimizer_sink;
+pub mod optimizer_metrics;
 
 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::{OptimizationMetrics, CompressorStats, optimize_record_with_metrics};
+pub use optimizer_metrics::MetricsCollector;