feat(realtime): improve append table lifecycle and query support

[lxy-9602]

Purpose

Linked issue: #158

This PR completes several follow-up capabilities for append-table real-time reads and writes:

• support batch-level statistics and predicate pushdown in the default Arrow RealtimeStore;
• support count readers and independently schedulable disk splits;
• make CommitWithProgress retries idempotent;
• preserve real-time progress across compaction and require context recreation after rollback;
• clean up expired and orphan real-time offset files;
• extend integration coverage for commit retry, compaction, rollback, schema rename, predicate pushdown, offset cleanup, and memory reclamation.

Tests

API and Format

• Adds the realtime.store.stats-mode option with none and full modes.
• Adds StatisticsMode to RealtimeStoreFactory::Create.
• Clarifies retry and rollback behavior in the public APIs.

Documentation

Generative AI tooling

Generated-by: OpenAI Codex (GPT-5)

[zjw1111]

Reviewed the realtime lifecycle / query changes. A few questions on the commit idempotency model and on StatisticsMode semantics, two perf points worth confirming, plus one test-coverage gap and three style nits. Inline.

[zjw1111]

+1