feat(memory): write-time importance scoring for improved retrieval

[bug-ops]

Summary

Add write-time importance scoring for memory retrieval, combining marker detection, content density, and role-based weighting. Importance score blends into the hybrid retrieval formula when enabled (default: disabled).

Addresses issue #2021 (MemOS: A Memory OS for AI Systems).

Changes

• Schema: Migration 039 adds importance_score REAL column with DEFAULT 0.5
• Importance Module: New semantic/importance.rs with 3-signal heuristic scorer
• Config: SemanticConfig.importance_enabled and importance_weight with validation
• Recall Path: Blends importance into final score when enabled, batch-increments access_count
• Save Path: Computes importance at write time in save_message_with_metadata()
• Bootstrap: Wires importance settings from config via .with_importance_options()
• Tests: 23 new unit/integration tests covering all code paths and edge cases

Key Features

• Backward Compatible: Feature is opt-in (disabled by default), old messages get neutral 0.5 score
• UTF-8 Safe: Fixed panic in marker detection via char_byte_boundary() helper
• Configurable Weights: Users can tune importance weight and enable/disable feature
• No New Dependencies: Pure Rust heuristic scoring
• --migrate-config Support: New config entries in default.toml for automatic user migration

Test Coverage

• 13 unit tests: importance computation, density curves, role adjustment, UTF-8 safety
• 10 integration tests: database operations, recall blending, access count tracking, migration compatibility

Pre-commit Checks

• ✅ cargo +nightly fmt --check
• ✅ cargo clippy --workspace --features full -- -D warnings
• ✅ All 6099+ tests passing

Closes #2021