refactor: add ECContext structure to encapsulate EC parameters

- Create ec_context.go with ECContext struct
- NewDefaultECContext() creates context with default 10+4 configuration
- Helper methods: CreateEncoder(), ToExt(), String()
- Foundation for cleaner function signatures
- No behavior change, still uses hardcoded 10+4

weed/storage/erasure_coding/ec_context.go

@@ -0,0 +1,44 @@
+package erasure_coding
+
+import (
+	"fmt"
+
+	"github.com/klauspost/reedsolomon"
+	"github.com/seaweedfs/seaweedfs/weed/storage/needle"
+)
+
+// ECContext encapsulates erasure coding parameters for encoding/decoding operations
+type ECContext struct {
+	DataShards   int
+	ParityShards int
+	TotalShards  int
+	Collection   string
+	VolumeId     needle.VolumeId
+}
+
+// NewDefaultECContext creates a context with default 10+4 shard configuration
+func NewDefaultECContext(collection string, volumeId needle.VolumeId) *ECContext {
+	return &ECContext{
+		DataShards:   DataShardsCount,
+		ParityShards: ParityShardsCount,
+		TotalShards:  TotalShardsCount,
+		Collection:   collection,
+		VolumeId:     volumeId,
+	}
+}
+
+// CreateEncoder creates a Reed-Solomon encoder for this context
+func (ctx *ECContext) CreateEncoder() (reedsolomon.Encoder, error) {
+	return reedsolomon.New(ctx.DataShards, ctx.ParityShards)
+}
+
+// ToExt returns the file extension for a given shard index
+func (ctx *ECContext) ToExt(shardIndex int) string {
+	return fmt.Sprintf(".ec%02d", shardIndex)
+}
+
+// String returns a human-readable representation of the EC configuration
+func (ctx *ECContext) String() string {
+	return fmt.Sprintf("%d+%d (total: %d)", ctx.DataShards, ctx.ParityShards, ctx.TotalShards)
+}
+