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.

server/internal/erasure/codec.go

@@ -0,0 +1,134 @@
+// Package erasure provides Reed–Solomon k-of-n shard encode/decode for spread payloads.
+package erasure
+
+import (
+	"bytes"
+	"crypto/sha256"
+	"encoding/hex"
+	"fmt"
+
+	"github.com/klauspost/reedsolomon"
+)
+
+const (
+	// SchemeReedSolomonV1 is the deploy-plan erasure scheme identifier.
+	SchemeReedSolomonV1 = "reed_solomon_v1"
+	// DefaultDataShards is the data shard count for spread payload encoding.
+	DefaultDataShards = 4
+	// DefaultParityShards is the parity shard count (any DefaultDataShards of total reconstruct).
+	DefaultParityShards = 2
+)
+
+// Params describes a Reed–Solomon split.
+type Params struct {
+	DataShards   int
+	ParityShards int
+}
+
+// DefaultParams returns the standard 4+2 erasure split.
+func DefaultParams() Params {
+	return Params{DataShards: DefaultDataShards, ParityShards: DefaultParityShards}
+}
+
+// MinShards returns the minimum shard count required for reconstruction.
+func (p Params) MinShards() int {
+	if p.DataShards <= 0 {
+		return 0
+	}
+	return p.DataShards
+}
+
+// TotalShards returns data + parity shard count.
+func (p Params) TotalShards() int {
+	return p.DataShards + p.ParityShards
+}
+
+// Normalize fills zero values with defaults and validates counts.
+func (p Params) Normalize() (Params, error) {
+	if p.DataShards <= 0 {
+		p.DataShards = DefaultDataShards
+	}
+	if p.ParityShards <= 0 {
+		p.ParityShards = DefaultParityShards
+	}
+	if p.DataShards < 1 || p.ParityShards < 1 {
+		return Params{}, fmt.Errorf("erasure: invalid shard counts data=%d parity=%d", p.DataShards, p.ParityShards)
+	}
+	if p.DataShards+p.ParityShards > 256 {
+		return Params{}, fmt.Errorf("erasure: too many shards")
+	}
+	return p, nil
+}
+
+// Encode splits payload into equal-sized Reed–Solomon shards.
+// The returned size is the original payload length (for Join on decode).
+func Encode(data []byte, p Params) ([][]byte, int, error) {
+	p, err := p.Normalize()
+	if err != nil {
+		return nil, 0, err
+	}
+	if len(data) == 0 {
+		return nil, 0, fmt.Errorf("erasure: empty payload")
+	}
+	origSize := len(data)
+	enc, err := reedsolomon.New(p.DataShards, p.ParityShards)
+	if err != nil {
+		return nil, 0, err
+	}
+	shards, err := enc.Split(data)
+	if err != nil {
+		return nil, 0, err
+	}
+	if err := enc.Encode(shards); err != nil {
+		return nil, 0, err
+	}
+	return shards, origSize, nil
+}
+
+// Decode reconstructs payload from at least MinShards() shards (nil entries allowed for missing).
+func Decode(shards [][]byte, payloadSize int, p Params) ([]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: shard verification failed")
+	}
+	if payloadSize <= 0 {
+		payloadSize = len(shards[0]) * p.DataShards
+	}
+	var buf bytes.Buffer
+	if err := enc.Join(&buf, shards, payloadSize); err != nil {
+		return nil, err
+	}
+	return buf.Bytes(), nil
+}
+
+// PayloadSHA256 returns hex SHA256 of the original payload.
+func PayloadSHA256(data []byte) string {
+	sum := sha256.Sum256(data)
+	return hex.EncodeToString(sum[:])
+}