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/store.go

@@ -0,0 +1,71 @@
+package erasure
+
+import (
+	"sync"
+)
+
+// ShardStore holds encoded shard bytes keyed by deploy-plan token.
+type ShardStore struct {
+	mu     sync.RWMutex
+	plans  map[string][][]byte
+	params map[string]Params
+}
+
+// NewShardStore creates an in-memory shard cache for public erasure-shard endpoints.
+func NewShardStore() *ShardStore {
+	return &ShardStore{
+		plans:  make(map[string][][]byte),
+		params: make(map[string]Params),
+	}
+}
+
+// Put stores shards for a token.
+func (s *ShardStore) Put(token string, p Params, shards [][]byte) {
+	if s == nil || token == "" || len(shards) == 0 {
+		return
+	}
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	cp := make([][]byte, len(shards))
+	for i, sh := range shards {
+		cp[i] = append([]byte(nil), sh...)
+	}
+	s.plans[token] = cp
+	s.params[token] = p
+}
+
+// Get returns one shard by index.
+func (s *ShardStore) Get(token string, index int) ([]byte, bool) {
+	if s == nil || token == "" {
+		return nil, false
+	}
+	s.mu.RLock()
+	defer s.mu.RUnlock()
+	shards, ok := s.plans[token]
+	if !ok || index < 0 || index >= len(shards) {
+		return nil, false
+	}
+	return append([]byte(nil), shards[index]...), true
+}
+
+// ParamsFor returns encoding params for a token.
+func (s *ShardStore) ParamsFor(token string) (Params, bool) {
+	if s == nil || token == "" {
+		return Params{}, false
+	}
+	s.mu.RLock()
+	defer s.mu.RUnlock()
+	p, ok := s.params[token]
+	return p, ok
+}
+
+// Delete removes a token (tests / TTL sweeps).
+func (s *ShardStore) Delete(token string) {
+	if s == nil || token == "" {
+		return
+	}
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	delete(s.plans, token)
+	delete(s.params, token)
+}