feat: initialize project structure and error types

- 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

errors.go

@@ -0,0 +1,22 @@
+package go_kv
+
+import "errors"
+
+// ErrCommitUnknown indicates the commit result is indeterminate: the WAL write
+// was attempted but the caller cannot assume the write definitely failed or
+// succeeded. The caller must query the commit state before retrying.
+var ErrCommitUnknown = errors.New("go-kv: commit result unknown")
+
+// ErrWriteStopped indicates the engine has entered a write-stopped state and
+// rejects all subsequent writes.
+var ErrWriteStopped = errors.New("go-kv: write stopped")
+
+// ErrSequenceExhausted indicates the sequence number space is exhausted. The
+// engine enters a terminal state requiring database migration or rebuild.
+var ErrSequenceExhausted = errors.New("go-kv: sequence exhausted")
+
+// ErrWALCorrupted indicates WAL data corruption was detected.
+var ErrWALCorrupted = errors.New("go-kv: WAL corrupted")
+
+// ErrInvalidConfig indicates an invalid configuration was provided.
+var ErrInvalidConfig = errors.New("go-kv: invalid config")