feat(hls): add native HLS (m3u8) download protocol

New hls protocol in internal/protocol/hls (registered ahead of http):
- master playlist variant picking (highest bandwidth), RFC3986 segment
  URL resolution, fMP4 init segments, EXT-X-BYTERANGE, EXT-X-GAP skip
- AES-128 decryption with mid-playlist key rotation and sequence-derived
  IVs; explicit errors for live streams, SAMPLE-AES and multi-period fMP4
- concurrent segment downloads with retry, journal-based resume across
  pause/app restarts, automatic in-order merge into a single .ts/.mp4
- best-effort Content-Length probing for determinate progress
- anti-hotlink Referer/Cookie/UA via task extra headers (Go transport
  does not filter them)

Core: fix SchemeFilter extension match for URLs with query strings
(also benefits .torrent links); Flutter: Protocol.hls enum, hls protocol
config model (prevents config round-trip wipe) and HLS settings block
with localized labels in all 22 locales.

Tests: parser/crypto unit tests, httptest e2e (Referer-enforced CDN,
encrypted streams, byterange, pause/resume, store/restore) and an
opt-in live-site acceptance gate (HLS_LIVE_URL).

ui/flutter/lib/api/model/downloader_config.dart

@@ -56,6 +56,7 @@ class ProtocolConfig {
   HttpConfig http = HttpConfig();
   BtConfig bt = BtConfig();
   Ed2kConfig ed2k = Ed2kConfig();
+  HlsConfig hls = HlsConfig();
 
   ProtocolConfig();
 
@@ -65,6 +66,25 @@ class ProtocolConfig {
   Map<String, dynamic> toJson() => _$ProtocolConfigToJson(this);
 }
 
+@JsonSerializable()
+class HlsConfig {
+  int segmentConnections;
+  int maxRetries;
+  int timeoutSeconds;
+  bool prefetchContentLength;
+
+  HlsConfig({
+    this.segmentConnections = 0,
+    this.maxRetries = 0,
+    this.timeoutSeconds = 0,
+    this.prefetchContentLength = false,
+  });
+
+  factory HlsConfig.fromJson(Map<String, dynamic> json) => _$HlsConfigFromJson(json);
+
+  Map<String, dynamic> toJson() => _$HlsConfigToJson(this);
+}
+
 @JsonSerializable()
 class HttpConfig {
   String userAgent;

ui/flutter/lib/api/model/downloader_config.g.dart

@@ -56,12 +56,28 @@ Map<String, dynamic> _$ApiServerConfigToJson(ApiServerConfig instance) => <Strin
 ProtocolConfig _$ProtocolConfigFromJson(Map<String, dynamic> json) => ProtocolConfig()
   ..http = HttpConfig.fromJson(json['http'] as Map<String, dynamic>)
   ..bt = BtConfig.fromJson(json['bt'] as Map<String, dynamic>)
-  ..ed2k = Ed2kConfig.fromJson(json['ed2k'] as Map<String, dynamic>);
+  ..ed2k = Ed2kConfig.fromJson(json['ed2k'] as Map<String, dynamic>)
+  ..hls = HlsConfig.fromJson(json['hls'] as Map<String, dynamic>);
 
 Map<String, dynamic> _$ProtocolConfigToJson(ProtocolConfig instance) => <String, dynamic>{
   'http': instance.http.toJson(),
   'bt': instance.bt.toJson(),
   'ed2k': instance.ed2k.toJson(),
+  'hls': instance.hls.toJson(),
+};
+
+HlsConfig _$HlsConfigFromJson(Map<String, dynamic> json) => HlsConfig(
+  segmentConnections: (json['segmentConnections'] as num?)?.toInt() ?? 0,
+  maxRetries: (json['maxRetries'] as num?)?.toInt() ?? 0,
+  timeoutSeconds: (json['timeoutSeconds'] as num?)?.toInt() ?? 0,
+  prefetchContentLength: json['prefetchContentLength'] as bool? ?? false,
+);
+
+Map<String, dynamic> _$HlsConfigToJson(HlsConfig instance) => <String, dynamic>{
+  'segmentConnections': instance.segmentConnections,
+  'maxRetries': instance.maxRetries,
+  'timeoutSeconds': instance.timeoutSeconds,
+  'prefetchContentLength': instance.prefetchContentLength,
 };
 
 HttpConfig _$HttpConfigFromJson(Map<String, dynamic> json) => HttpConfig(

ui/flutter/lib/api/model/task.dart

@@ -6,7 +6,7 @@ part 'task.g.dart';
 
 enum Status { ready, running, pause, wait, error, done }
 
-enum Protocol { http, bt, ed2k }
+enum Protocol { http, bt, ed2k, hls }
 
 // ExtractStatus enum matching Go backend
 enum ExtractStatus {

ui/flutter/lib/api/model/task.g.dart

@@ -38,7 +38,7 @@ const _$StatusEnumMap = {
   Status.done: 'done',
 };
 
-const _$ProtocolEnumMap = {Protocol.http: 'http', Protocol.bt: 'bt', Protocol.ed2k: 'ed2k'};
+const _$ProtocolEnumMap = {Protocol.http: 'http', Protocol.bt: 'bt', Protocol.ed2k: 'ed2k', Protocol.hls: 'hls'};
 
 Progress _$ProgressFromJson(Map<String, dynamic> json) => Progress(
   used: (json['used'] as num).toInt(),

ui/flutter/lib/api/model/task_stats.dart

@@ -55,7 +55,7 @@ sealed class TaskStats {
       Protocol.http => HttpTaskStats.fromJson(snapshot),
       Protocol.bt => BtTaskStats.fromJson(snapshot: snapshot, runtime: runtime),
       Protocol.ed2k => Ed2kTaskStats.fromJson(snapshot: snapshot, runtime: runtime),
-      null => null,
+      Protocol.hls || null => null,
     };
   }
 }