chore: remove ~50k lines of unreachable dead code (#8913)

* chore: remove unreachable dead code across the codebase

Remove ~50,000 lines of unreachable code identified by static analysis.

Major removals:
- weed/filer/redis_lua: entire unused Redis Lua filer store implementation
- weed/wdclient/net2, resource_pool: unused connection/resource pool packages
- weed/plugin/worker/lifecycle: unused lifecycle plugin worker
- weed/s3api: unused S3 policy templates, presigned URL IAM, streaming copy,
  multipart IAM, key rotation, and various SSE helper functions
- weed/mq/kafka: unused partition mapping, compression, schema, and protocol functions
- weed/mq/offset: unused SQL storage and migration code
- weed/worker: unused registry, task, and monitoring functions
- weed/query: unused SQL engine, parquet scanner, and type functions
- weed/shell: unused EC proportional rebalance functions
- weed/storage/erasure_coding/distribution: unused distribution analysis functions
- Individual unreachable functions removed from 150+ files across admin,
  credential, filer, iam, kms, mount, mq, operation, pb, s3api, server,
  shell, storage, topology, and util packages

* fix(s3): reset shared memory store in IAM test to prevent flaky failure

TestLoadIAMManagerFromConfig_EmptyConfigWithFallbackKey was flaky because
the MemoryStore credential backend is a singleton registered via init().
Earlier tests that create anonymous identities pollute the shared store,
causing LookupAnonymous() to unexpectedly return true.

Fix by calling Reset() on the memory store before the test runs.

* style: run gofmt on changed files

* fix: restore KMS functions used by integration tests

* fix(plugin): prevent panic on send to closed worker session channel

The Plugin.sendToWorker method could panic with "send on closed channel"
when a worker disconnected while a message was being sent. The race was
between streamSession.close() closing the outgoing channel and sendToWorker
writing to it concurrently.

Add a done channel to streamSession that is closed before the outgoing
channel, and check it in sendToWorker's select to safely detect closed
sessions without panicking.

weed/storage/erasure_coding/placement/placement.go

@@ -64,18 +64,6 @@ type PlacementRequest struct {
 	PreferDifferentRacks bool
 }
 
-// DefaultPlacementRequest returns the default placement configuration
-func DefaultPlacementRequest() PlacementRequest {
-	return PlacementRequest{
-		ShardsNeeded:           14,
-		MaxShardsPerServer:     0,
-		MaxShardsPerRack:       0,
-		MaxTaskLoad:            5,
-		PreferDifferentServers: true,
-		PreferDifferentRacks:   true,
-	}
-}
-
 // PlacementResult contains the selected destinations for EC shards
 type PlacementResult struct {
 	SelectedDisks []*DiskCandidate
@@ -270,15 +258,6 @@ func groupDisksByRack(disks []*DiskCandidate) map[string][]*DiskCandidate {
 	return result
 }
 
-// groupDisksByServer groups disks by their server
-func groupDisksByServer(disks []*DiskCandidate) map[string][]*DiskCandidate {
-	result := make(map[string][]*DiskCandidate)
-	for _, disk := range disks {
-		result[disk.NodeID] = append(result[disk.NodeID], disk)
-	}
-	return result
-}
-
 // getRackKey returns the unique key for a rack (dc:rack)
 func getRackKey(disk *DiskCandidate) string {
 	return fmt.Sprintf("%s:%s", disk.DataCenter, disk.Rack)
@@ -393,28 +372,3 @@ func addDiskToResult(result *PlacementResult, disk *DiskCandidate,
 	result.ShardsPerRack[rackKey]++
 	result.ShardsPerDC[disk.DataCenter]++
 }
-
-// VerifySpread checks if the placement result meets diversity requirements
-func VerifySpread(result *PlacementResult, minServers, minRacks int) error {
-	if result.ServersUsed < minServers {
-		return fmt.Errorf("only %d servers used, need at least %d", result.ServersUsed, minServers)
-	}
-	if result.RacksUsed < minRacks {
-		return fmt.Errorf("only %d racks used, need at least %d", result.RacksUsed, minRacks)
-	}
-	return nil
-}
-
-// CalculateIdealDistribution returns the ideal number of shards per server
-// when we have a certain number of shards and servers
-func CalculateIdealDistribution(totalShards, numServers int) (min, max int) {
-	if numServers <= 0 {
-		return 0, totalShards
-	}
-	min = totalShards / numServers
-	max = min
-	if totalShards%numServers != 0 {
-		max = min + 1
-	}
-	return
-}