[BUG] Percolator commit accepts commitVersion earlier than startVersion

[feichai0017]

🐛 Bug Description

percolator.Commit currently accepts commitVersion < startVersion and persists it.
This violates MVCC ordering and allows values to become visible before the transaction start timestamp.

Code path:

• percolator/txn.go:88
• percolator/txn.go:279
• percolator/txn.go:299

🛠 Reproduction Steps

1. Prewrite key k with startVersion=20.
2. Commit with commitVersion=10.
3. Read key at readTs=15.

📋 Expected Behavior

Commit should reject requests where commitVersion < startVersion.
Read at ts=15 must not observe data from transaction startTs=20.

📸 Screenshots / Logs

Observed output:

• commit keyError: <nil>
• write found, commitTs=10, startTs=20
• read@15 value="v" err=<nil>

💻 Environment

• OS: macOS 26.2 (arm64)
• Go Version: go1.24.4
• NoKV Version: current main (local HEAD on 2026-02-11)

🧐 Additional Context

This issue can break snapshot visibility guarantees for distributed transactions.