Phase 2a: buffer state under one lock, atomic tuner limit, context-driven external buffer

- src/buffer_state.go replaces the two sync.Maps plus a global RWMutex with
  one bufferMu guarding a map of *Playlist. Each stream has one shared
  bufferStream (URL, folder, status, client count, error, cancel hook);
  downloaders keep a private ThisStream and publish through helpers.
  Playlist.Clients / ThisClient / ClientConnection are gone: there was one
  client counter per stream in two places that could disagree.
- bufferAcquireStream does the tuner check and the registration under the
  same lock, so two clients tuning at once cannot both pass the limit.
  bufferReleaseClient removes the stream when the last client leaves,
  cancels its process and deletes its segment folder.
- bufferingStream rewritten: waits on r.Context() instead of the deprecated
  CloseNotifier, sets Content-Type before WriteHeader (the old code set
  headers after and one was literally named "Content-Length:"), flushes
  each segment to the client, no defer inside the segment loop.
- connectToStreamingServer: shared streamHTTPClient with dial, TLS and
  response-header timeouts (no overall timeout, bodies are endless); the
  deferred Body/segment closes inside the redirect and read loops are now
  explicit closes, so a multi-hour stream no longer accumulates them.
- thirdPartyBuffer rewritten around exec.CommandContext: the process is
  killed when the last client leaves or when no usable data arrives within
  20 s (time.AfterFunc watchdog, no leaked goroutine); cmd.Start error is
  checked; no panic; one file handle per segment. ffmpeg and VLC command
  lines come from buildFFmpegArgs / buildVLCArgs, which are unit tested.
- Data.Cache.StreamingURLS is guarded by streamingURLsMu and persisted
  from a snapshot.
- Tests (src/buffer_test.go, run with -race): restream shares one provider
  connection, six concurrent tunes against a tuner limit of two, 50-way
  acquire/release contention, cleanup and cancel on last release.

.gitignore

@@ -12,3 +12,4 @@ de.json
 agent.md
 skill.md
 node_modules/
+.claude/