[Enhancement] Avoid per-message getBytes() for topic and CRC32 property key on the broker write path

### Before Creating the Enhancement Request

- [x] I have confirmed that this should be classified as an enhancement rather than a bug/feature.

### Summary

Cache the topic bytes in `MessageExtEncoder` and the CRC32 property-key bytes in `MessageDecoder` to avoid two per-message `String.getBytes()` allocations on the broker write path.

### Motivation

- `MessageExtEncoder.encode()` / `encodeWithoutProperties()` call `msgInner.getTopic().getBytes(UTF_8)` for every message. Broker traffic is highly repetitive per thread, and the encoder instance is already `ThreadLocal`, so the same topic string is re-encoded over and over.
- `MessageDecoder.createCrc32()` re-encodes the constant property key `MessageConst.PROPERTY_CRC32` on every message when CRC32 stamping is enabled.

### Solution

- `MessageExtEncoder`: keep a single-slot `cachedTopic`/`cachedTopicData` pair (no synchronization needed — the encoder is `ThreadLocal`). Hit returns the cached bytes; miss falls back to `getBytes` and refreshes the slot, so mixed-topic traffic is never worse than today apart from one string comparison.
- `MessageDecoder`: pre-encode `PROPERTY_CRC32` into a `private static final byte[]`.

### Verification

- `AppendCallbackTest` 4/4, `AppendPropCRCTest` 2/2 (covers the CRC32 constant path), `LmqDispatchTest` 5/5, `CompactionLogTest` 4/4, `MessageDecoderTest` 7/7; checkstyle clean.
- 4-node cluster A/B (256-thread sync producer, 1KB, per-arm clean store + restart): broker TPS and young GC per million messages are flat versus baseline — the saving (one short `byte[]` per message) is below GC-count resolution at this load, so this is a cleanup-level allocation reduction, not a measurable throughput win.

Start by reading MessageExtEncoder.encode() and encodeWithoutProperties(), then inspect MessageDecoder.createCrc32() and the existing verification tests. Use AppendCallbackTest, AppendPropCRCTest, LmqDispatchTest, CompactionLogTest, and MessageDecoderTest to confirm behavior and checkstyle; done means the repeated byte conversions are cached without changing message encoding or CRC32 results.

Written by the indexing model from the issue text.