Implement Stage 11 importance scoring

[aarambh-darshan]

Summary

Implements Stage 11 importance scoring and promotion for Manas.

What Changed

• Added manas-learn/src/importance.rs with weighted importance scoring.
• Added public Stage 11 thresholds:
  • OPEN_TO_GUARDED_IMPORTANCE = 0.50
  • GUARDED_TO_FROZEN_IMPORTANCE = 0.85
• Computes neuron importance from:
  • activation frequency
  • recency
  • weight magnitude
  • smooth age grace
• Replaced activation-count-only trainer promotion with importance-driven promotion.
• Preserved monotonic one-level promotion behavior:
  • Open -> Guarded
  • Guarded -> Frozen
  • Frozen never downgrades
• Added learning metadata stamping for born_at and last_activated.
• Preserved existing ProtectionReport and LearnReport semantics.
• Added persistence coverage proving importance_score, born_at, last_activated, and activation_count survive .manas save/load.
• Updated README, ROADMAP, CHANGELOG, and ARCHITECTURE to mark Stage 11 complete and Stage 12 next.

Tests

• cargo test -p manas-learn importance
• cargo test -p manas-learn protection
• cargo test -p manas-store
• cargo test -p manas-learn anti_forgetting
• cargo test --workspace
• cargo clippy --workspace --all-targets -- -D warnings
• cargo fmt --all -- --check
• git diff --check
• cargo build --workspace --release

Notes

No .manas format bump is needed. Stage 11 uses fields already present in the persisted neuron layout.