fix: cancel consolidation across session reloads

[gpmarques]

Summary

Reloading or replacing a session during active consolidation reproduced a stale-runtime append: late observer/reflector output could write through the old extension instance, while reflection work was lost instead of being retried by the fresh session.

This change:

• cancels observer, reflector, dropper, model resolution, and deferred compaction work on shutdown
• guards every post-await result and ledger append with a session/runtime generation
• lets a fresh runtime retry consolidation without accepting stale output

Verification

• 265 native tests passed
• typecheck passed