Replace polling loop (Collect + Sleep) with proper batch collection:
- Block on first request via channel receive
- Start 500µs timer for collection window
- Accumulate requests until timer fires or batch reaches 32KB
- Shutdown handling at all blocking points
Design doc §3.2.5: '等待组提交触发（500µs 或 32KB，先到者触发）'
Add GroupCommitDelay config field (default 500µs, must be > 0 and < 10ms).
- go.mod with github.com/dailz/go-kv, Go 1.26.3, testify
- config/config.go with WalConfig, Validate() with checked arithmetic
- errors.go with sentinel errors (ErrCommitUnknown, ErrWriteStopped, etc.)
- wal/constants.go with all WAL format constants and enums
- wal/header.go with WAL File Header encode/decode (CRC32 IEEE)
- wal/record.go with Physical Record codec, block boundary, SplitIntoRecords
- wal/entry.go with WAL Entry codec (varint keys/values, OpType, ValueKind)
- wal/sequence.go with SequenceManager (atomic, CAS, overflow-safe)
- manifest/manifest.go with MANIFEST stub (Load/Save atomic)
- manifest/current.go with CURRENT file (WriteCurrent/ReadCurrent)
- Comprehensive tests for all modules
- .golangci.yml configuration
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