feat: M3.8.1 phase 4a — TextCompressor + env config (12 tests)

TextCompressor (320 LOC, 10 tests):
- Token importance scoring with lazy_static STOP_WORDS
- Keeps: high-entropy tokens (IDs, hashes, error codes, numbers, symbols)
- Drops: stop words, filler words, low-information prose
- ID detection: UUID, SHA256, session IDs, underscored patterns
- Error marker detection: error, exception, panic, fail, warn, critical
- Configurable compression ratio (default 40% token retention)

ContextOptimizerConfig::from_env() (2 tests):
- MEM_CONTEXT_OPTIMIZER (on/off)
- MEM_MAGIKA_ENABLED, MEM_MAGIKA_THRESHOLD
- MEM_COMPRESS_JSON, MEM_COMPRESS_LOGS, MEM_COMPRESS_CODE, MEM_COMPRESS_DIFF, MEM_COMPRESS_TEXT
- MEM_TOKEN_BUDGET, MEM_CCR_ENABLED

ContextOptimizer::from_env() factory method

62 optimizer tests total:
Phase 1 (17) + Phase 2 (15) + Phase 3 (18) + Phase 4a (12) = 62 passing

crates/mem-core/Cargo.toml

@@ -21,3 +21,4 @@ ort = { version = "2.0.0-rc.12", default-features = true }
 regex = "1.10"
 once_cell = "1.19"
 indexmap = "2.0"
+lazy_static = "1.4"