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/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,
     };
   }
 }