feat: Implement Monomorphic Inline Cache (MIC) for message sends

Add one-slot inline cache to MessageNode for fast method dispatch:
- cachedClass: stores the last receiver class seen
- cachedMethod: stores the method for that class
- callCount: tracks call site usage (for future PIC)

Update WorkFrame to include msgNode reference for wfSendMessage.
Update VM to check cache before full method lookup:
- Cache hit: use cached method directly (bypass lookupMethod)
- Cache miss: do full lookup and update cache

The MIC is populated when message sends are evaluated from
MessageNode (nkMessage). Continuation-based sends don't
have cache access yet (msgNode is nil).

Note: Performance impact needs evaluation - may need tuning.

Author: gokr