fix(stream): no anunciar un total falso mientras el remux crece (loop de re-seek)

serveGrowing anunciaba en Content-Range total = EstimatedSize() = el tamaño
del MKV fuente mientras ffmpeg aún corría. Pero el fMP4 resultante no mide
eso (el audio re-encodea a AAC y la fragmentación cambian el byte count), así
que el <video> nativo mapeaba su timeline sobre una longitud falsa, pedía
offsets que no cuadraban, re-seekeaba y reabría la conexión cientos de veces
por segundo (el loop de reproducción remux).

Mientras crece (!Final) la longitud real es DESCONOCIDA: ahora se sirve
Content-Range "bytes start-end/*" (RFC 7233 §4.2) sin Content-Length, y el
cliente lee secuencial en vez de re-seekear. Cuando ffmpeg termina, el tamaño
real se conoce y se anuncia como antes. El 416 y el Content-Length del HEAD
solo cuando el total es real (final).

internal/engine/stream_growing_test.go

@@ -135,9 +135,12 @@ func TestServeGrowing_BoundedRange(t *testing.T) {
 	}
 }
 
-func TestServeGrowing_EstimateUsedWhileNotFinal(t *testing.T) {
-	// Not final: only 8 bytes produced, but estimate says 100. The advertised
-	// total is the estimate (scrubber timeline); body is what exists so far.
+func TestServeGrowing_UnknownTotalWhileNotFinal(t *testing.T) {
+	// Not final: only 8 bytes produced, estimate says 100. The instance length
+	// is genuinely unknown while the remux grows, so we advertise "/*" (RFC 7233
+	// §4.2) instead of a total the native player would map its timeline onto and
+	// re-seek against (the playback loop). The estimate is only an upper-bound
+	// hint for `end`; body is what exists so far.
 	src := &fakeGrowing{data: []byte("01234567"), final: false, est: 100}
 	ss := &StreamServer{}
 
@@ -149,8 +152,8 @@ func TestServeGrowing_EstimateUsedWhileNotFinal(t *testing.T) {
 	if res.StatusCode != http.StatusPartialContent {
 		t.Fatalf("status = %d, want 206", res.StatusCode)
 	}
-	if got := res.Header.Get("Content-Range"); got != "bytes 0-99/100" {
-		t.Errorf("Content-Range = %q, want bytes 0-99/100 (estimate)", got)
+	if got := res.Header.Get("Content-Range"); got != "bytes 0-99/*" {
+		t.Errorf("Content-Range = %q, want bytes 0-99/* (unknown total)", got)
 	}
 	// Not final → no exact Content-Length (chunked) so we never promise bytes
 	// a still-running remux might not produce.
@@ -163,6 +166,8 @@ func TestServeGrowing_EstimateUsedWhileNotFinal(t *testing.T) {
 }
 
 func TestServeGrowing_HeadProbe(t *testing.T) {
+	// HEAD while growing: total is unknown, so no Content-Length is promised
+	// (advertising the estimate is the bug this fix removes).
 	src := &fakeGrowing{data: make([]byte, 0), final: false, est: 4242}
 	ss := &StreamServer{}
 
@@ -174,14 +179,32 @@ func TestServeGrowing_HeadProbe(t *testing.T) {
 	if res.StatusCode != http.StatusOK {
 		t.Fatalf("HEAD status = %d, want 200", res.StatusCode)
 	}
-	if got := res.Header.Get("Content-Length"); got != "4242" {
-		t.Errorf("HEAD Content-Length = %q, want 4242", got)
+	if got := res.Header.Get("Content-Length"); got != "" {
+		t.Errorf("HEAD Content-Length = %q, want empty (unknown total while growing)", got)
 	}
 	if rec.Body.Len() != 0 {
 		t.Errorf("HEAD body = %d bytes, want 0", rec.Body.Len())
 	}
 }
 
+func TestServeGrowing_HeadProbeFinal(t *testing.T) {
+	// HEAD once final: the true total IS known, so advertise it.
+	src := &fakeGrowing{data: make([]byte, 4242), final: true}
+	ss := &StreamServer{}
+
+	req := httptest.NewRequest(http.MethodHead, "/stream", nil)
+	rec := httptest.NewRecorder()
+	ss.serveGrowing(rec, req, src)
+
+	res := rec.Result()
+	if res.StatusCode != http.StatusOK {
+		t.Fatalf("HEAD status = %d, want 200", res.StatusCode)
+	}
+	if got := res.Header.Get("Content-Length"); got != "4242" {
+		t.Errorf("HEAD Content-Length = %q, want 4242 (final size known)", got)
+	}
+}
+
 func TestServeGrowing_RangeBeyondTotal(t *testing.T) {
 	src := &fakeGrowing{data: []byte("0123456789"), final: true}
 	ss := &StreamServer{}

internal/engine/stream_server.go

@@ -1467,25 +1467,38 @@ func (ss *StreamServer) serveGrowing(w http.ResponseWriter, r *http.Request, src
 	w.Header().Set("Content-Type", "video/mp4")
 	w.Header().Set("Content-Disposition", fmt.Sprintf("inline; filename=%q", src.FileName()))
 
-	// Total to advertise: exact when ffmpeg has exited, else the estimate.
-	total := src.EstimatedSize()
-	if src.Final() {
-		total = src.Size()
+	// The instance length is KNOWN only once ffmpeg has exited. While the remux
+	// is still growing, the final size is genuinely unknown — the source MKV
+	// size is NOT it (the audio re-encode to AAC + fMP4 fragmentation change the
+	// byte count). Advertising that wrong total made the native <video> map its
+	// timeline onto a bogus length, request byte offsets that didn't line up,
+	// re-seek, and reopen the connection hundreds of times a second (the remux
+	// playback loop). Per RFC 7233 §4.2 we now send "/*" (unknown total) while
+	// growing, so the player streams sequentially instead of re-seeking against
+	// a fake size. `end` uses the estimate only as an upper-bound hint.
+	final := src.Final()
+	total := src.Size()
+	if !final {
+		total = src.EstimatedSize()
 	}
 	if total <= 0 {
 		total = src.Size()
 	}
 
 	start, explicitEnd := parseByteRange(r.Header.Get("Range"))
-	if total > 0 && start >= total {
-		// Range beyond what we expect to produce — let the browser recover.
+	// A 416 is only sound against a KNOWN total. While growing we can't say a
+	// start is unsatisfiable (more bytes are still coming), so only guard when
+	// final.
+	if final && total > 0 && start >= total {
+		// Range beyond the real end — let the browser recover.
 		w.Header().Set("Content-Range", fmt.Sprintf("bytes */%d", total))
 		http.Error(w, "range not satisfiable", http.StatusRequestedRangeNotSatisfiable)
 		return
 	}
 
 	if r.Method == http.MethodHead {
-		if total > 0 {
+		// Only promise a length we actually know (final). While growing, omit it.
+		if final && total > 0 {
 			w.Header().Set("Content-Length", strconv.FormatInt(total, 10))
 		}
 		w.WriteHeader(http.StatusOK)
@@ -1496,13 +1509,23 @@ func (ss *StreamServer) serveGrowing(w http.ResponseWriter, r *http.Request, src
 	if explicitEnd >= 0 && explicitEnd < end {
 		end = explicitEnd
 	}
-	if total > 0 {
-		w.Header().Set("Content-Range", fmt.Sprintf("bytes %d-%d/%d", start, end, total))
+	if end < start {
+		end = start
 	}
-	// Exact Content-Length only when the source is final (true size known) so
-	// we never promise bytes a still-running remux might not produce.
-	if src.Final() && explicitEnd < 0 {
-		w.Header().Set("Content-Length", strconv.FormatInt(src.Size()-start, 10))
+	if final {
+		if total > 0 {
+			w.Header().Set("Content-Range", fmt.Sprintf("bytes %d-%d/%d", start, end, total))
+		}
+		// Exact Content-Length only when final (true size known) so we never
+		// promise bytes a still-running remux might not produce.
+		if explicitEnd < 0 {
+			w.Header().Set("Content-Length", strconv.FormatInt(src.Size()-start, 10))
+		}
+	} else {
+		// Growing: honest "unknown total" so the player doesn't re-seek against
+		// a wrong size. No Content-Length (chunked) — bytes flow as ffmpeg makes
+		// them and the read loop below blocks at the live edge.
+		w.Header().Set("Content-Range", fmt.Sprintf("bytes %d-%d/*", start, end))
 	}
 	w.WriteHeader(http.StatusPartialContent)