feat: adopt ELRS USB CRSF frame format, add skeleton modules

Switch CRC from CCITT (0x07) to DVB-S2 (0xD5) to match ELRS.
Adopt ELRS USB frame format: [addr][length][type][payload][crc].
Update frame type constants to match current ELRS protocol values.

New skeleton modules (stub implementations with TODO comments):
  - crsf_telemetry.h/.c: telemetry decoders (GPS, battery, link..)
  - crsf_stream.h/.c: incremental streaming frame reader
  - crsf_param.h/.c: parameter protocol (read/write/set_power)

Retained old frame format as *_legacy functions for backward
compatibility with existing telemetry tool and tests.

Add cel_serial_find_elrs_port() and cel_serial_open_probe()
stubs to serial module for port auto-detection and baud probing.

celrs/crsf_telemetry.h

@@ -0,0 +1,145 @@
+#pragma once
+#include <stdint.h>
+#include "celrs/crsf.h"
+
+/* Telemetry frame types */
+typedef enum {
+    CEL_TELEM_GPS        = 0,
+    CEL_TELEM_VARIO      = 1,
+    CEL_TELEM_BATTERY    = 2,
+    CEL_TELEM_BARO       = 3,
+    CEL_TELEM_AIRSPEED   = 4,
+    CEL_TELEM_HEARTBEAT  = 5,
+    CEL_TELEM_RPM        = 6,
+    CEL_TELEM_TEMP       = 7,
+    CEL_TELEM_VOLTAGES   = 8,
+    CEL_TELEM_ESC        = 9,
+    CEL_TELEM_LINK       = 10,
+    CEL_TELEM_ATTITUDE   = 11,
+    CEL_TELEM_FLIGHT_MODE = 12,
+    CEL_TELEM_PARAM      = 13,
+    CEL_TELEM_UNKNOWN    = 14,
+} cel_telem_type;
+
+/* GPS telemetry */
+typedef struct {
+    float   latitude_deg;
+    float   longitude_deg;
+    float   groundspeed_kmh;
+    float   heading_deg;
+    float   altitude_m;
+    uint8_t satellites;
+} cel_telem_gps;
+
+/* Vario (vertical speed) */
+typedef struct {
+    int16_t vertical_speed_cms;
+} cel_telem_vario;
+
+/* Battery sensor */
+typedef struct {
+    float   voltage_v;
+    float   current_a;
+    uint32_t capacity_mah;
+    uint8_t  remaining_pct;
+} cel_telem_battery;
+
+/* Barometric altitude */
+typedef struct {
+    float   altitude_m;
+    int16_t vertical_speed_cms;
+} cel_telem_baro;
+
+/* Airspeed */
+typedef struct {
+    float speed_kmh;
+} cel_telem_airspeed;
+
+/* Heartbeat */
+typedef struct {
+    uint16_t origin_addr;
+} cel_telem_heartbeat;
+
+/* RPM sensor (up to 8 values) */
+typedef struct {
+    uint8_t source;
+    int32_t values[8];
+    uint8_t count;
+} cel_telem_rpm;
+
+/* Temperature sensor (up to 8 values, deg C) */
+typedef struct {
+    uint8_t source;
+    float   values[8];
+    uint8_t count;
+} cel_telem_temp;
+
+/* Voltage cells (up to 8 values, volts) */
+typedef struct {
+    uint8_t source;
+    float   values[8];
+    uint8_t count;
+} cel_telem_voltages;
+
+/* ESC telemetry (BLHeli/KISS passthrough) */
+typedef struct {
+    float   voltage_v;
+    float   current_a;
+    uint16_t consumed_mah;
+    uint32_t rpm;
+    uint8_t  temp_c;
+} cel_telem_esc;
+
+/* Link statistics */
+typedef struct {
+    uint8_t  uplink_rssi1;
+    uint8_t  uplink_rssi2;
+    uint8_t  uplink_quality;
+    int8_t   uplink_snr;
+    uint8_t  active_antenna;
+    uint8_t  rf_mode;
+    uint8_t  uplink_power;
+    uint8_t  downlink_rssi;
+    uint8_t  downlink_qual;
+    int8_t   downlink_snr;
+} cel_telem_link;
+
+/* Attitude (pitch/roll/yaw in radians) */
+typedef struct {
+    float pitch_rad;
+    float roll_rad;
+    float yaw_rad;
+} cel_telem_attitude;
+
+/* Flight mode string */
+typedef struct {
+    char name[32];
+} cel_telem_flight_mode;
+
+/* Union of all telemetry data types */
+typedef union {
+    cel_telem_gps        gps;
+    cel_telem_vario      vario;
+    cel_telem_battery    battery;
+    cel_telem_baro       baro;
+    cel_telem_airspeed   airspeed;
+    cel_telem_heartbeat  heartbeat;
+    cel_telem_rpm        rpm;
+    cel_telem_temp       temp;
+    cel_telem_voltages   voltages;
+    cel_telem_esc        esc;
+    cel_telem_link       link;
+    cel_telem_attitude   attitude;
+    cel_telem_flight_mode flight_mode;
+} cel_telem_data;
+
+/* Parsed telemetry frame */
+typedef struct {
+    cel_telem_type type;
+    cel_telem_data data;
+} cel_telemetry;
+
+/* Parse a CRSF frame's payload into a telemetry struct.
+   Returns 0 on success, -1 on unsupported type or bad payload length. */
+int cel_crsf_telemetry_parse(cel_crsf_frame const* frame,
+                              cel_telemetry* out);