feat: GET /api/color/latest, the fast color channel (no align)

JPEG of the latest color frame at the camera rate, not aligned to depth.
after=N waits up to timeout_ms (default 1000, max 5000) for a frame numbered
above N, so a client reads every frame without polling on empty; 504 when
none arrives. Headers: X-Frame-Seq, X-Camera-Timestamp-Ms (camera clock),
X-Camera-Timestamp-Domain, X-Host-Timestamp (arrival on the host).
Each color frame is encoded once for all clients (EncodedColor, own cache of
4: the color rate would flush the aligned frames' cache). New timing phase
jpeg_color. /api/rgb is unchanged: still the color of the aligned frame, the
same frame as /api/depth (README says so next to the new row).
color_frame() without after waits for the very first frame after open.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

README.md

@@ -165,7 +165,8 @@ suo `.env` l'URL della camera con `http://192.168.123.99:8878`.
 | `/api/open` | POST | Apre camera per `device_index` (di default partita in auto-open) |
 | `/api/close` | POST | Chiude camera |
 | `/api/capture` | GET | RGB+depth in JSON (base64) |
-| `/api/rgb` | GET | RGB come JPEG |
+| `/api/rgb` | GET | RGB come JPEG (colore del frame allineato: stesso frame di `/api/depth`) |
+| `/api/color/latest` | GET | **Canale colore veloce**: JPEG dell'ultimo frame colore, **senza align**, al ritmo della camera. `after=N` aspetta un frame numerato oltre N (fino a `timeout_ms`, default 1000, max 5000; 504 se non arriva). Header `X-Frame-Seq`, `X-Camera-Timestamp-Ms` (orologio della camera), `X-Camera-Timestamp-Domain`, `X-Host-Timestamp` (arrivo sull'host, s). Non va usato insieme a `/api/depth`: non è lo stesso frame |
 | `/api/depth` | GET | Depth come PNG 16-bit (mm) |
 | `/api/depth/color` | GET | Depth colormap JET come JPEG |
 | `/api/intrinsics` | GET | Calibrazione completa (`ir`+`rgb`+`extrinsics_ir_to_rgb`) |