fix: replace broken KV cache cleanup

[myungjin]

Description

Replace seqno-offset cache cleanup with activity-based approach using exponential moving average (EMA) to prevent memory leaks and premature evictions in pipelined execution with replicas.

Key changes:

• Track last access timestamp per cache instead of seqno ordering
• Adaptive timeout: EMA × multiplier × max_inflight for pipeline-aware cleanup
• Handles out-of-order completion and replica stages correctly
• Silent operation with zero logging overhead

Configuration:

• ema_alpha=0.5, timeout_multiplier=10
• min_timeout=0.5s, max_timeout=30.0s

Type of Change

• Bug Fix
• New Feature
• Breaking Change
• Refactor
• Documentation
• Other (please describe)

Checklist

• I have read the contributing guidelines
• Existing issues have been referenced (where applicable)
• I have verified this change is not present in other open pull requests
• Functionality is documented
• All code style checks pass
• New code contribution is covered by automated tests
• All new and existing tests pass