Add erasure-coded multi-lane spread foundation.

Server Reed-Solomon 4+2 shard encode on deploy plans when erasure_lanes_enabled; agents reassemble from parallel lane URLs as staging fallback with BGP/Path Tracer hints.

agent/deploy/erasure_codec.go

@@ -0,0 +1,82 @@
+package deploy
+
+import (
+	"bytes"
+	"fmt"
+
+	"github.com/klauspost/reedsolomon"
+)
+
+const (
+	erasureSchemeReedSolomonV1 = "reed_solomon_v1"
+	erasureDefaultDataShards   = 4
+	erasureDefaultParityShards = 2
+)
+
+type erasureParams struct {
+	DataShards   int
+	ParityShards int
+}
+
+func (p erasureParams) normalize() (erasureParams, error) {
+	if p.DataShards <= 0 {
+		p.DataShards = erasureDefaultDataShards
+	}
+	if p.ParityShards <= 0 {
+		p.ParityShards = erasureDefaultParityShards
+	}
+	if p.DataShards < 1 || p.ParityShards < 1 {
+		return erasureParams{}, fmt.Errorf("erasure: invalid shard counts")
+	}
+	return p, nil
+}
+
+func (p erasureParams) minShards() int {
+	return p.DataShards
+}
+
+func (p erasureParams) totalShards() int {
+	return p.DataShards + p.ParityShards
+}
+
+// decodeErasureShards reconstructs payload from Reed–Solomon shards (nil = missing).
+func decodeErasureShards(shards [][]byte, payloadSize int, p erasureParams) ([]byte, error) {
+	p, err := p.normalize()
+	if err != nil {
+		return nil, err
+	}
+	if len(shards) < p.totalShards() {
+		return nil, fmt.Errorf("erasure: shard slice too short")
+	}
+	present := 0
+	for i := 0; i < p.totalShards(); i++ {
+		if len(shards[i]) > 0 {
+			present++
+		}
+	}
+	if present < p.minShards() {
+		return nil, fmt.Errorf("erasure: need %d shards, have %d", p.minShards(), present)
+	}
+	enc, err := reedsolomon.New(p.DataShards, p.ParityShards)
+	if err != nil {
+		return nil, err
+	}
+	if err := enc.Reconstruct(shards); err != nil {
+		return nil, err
+	}
+	ok, err := enc.Verify(shards)
+	if err != nil {
+		return nil, err
+	}
+	if !ok {
+		return nil, fmt.Errorf("erasure: verification failed")
+	}
+	if payloadSize <= 0 {
+		payloadSize = len(shards[0]) * p.DataShards
+	}
+	var out bytes.Buffer
+	if err := enc.Join(&out, shards, payloadSize); err != nil {
+		return nil, err
+	}
+	return out.Bytes(), nil
+}