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.

README.md

@@ -74,19 +74,22 @@ mock files are excluded. Requires GCC or Clang with gcov support, and
 
 ```
 celrs/
-  crsf.h / crsf.c      CRSF protocol: CRC8, frame parse/build
-  serial.h / serial.c  Serial port abstraction (Win/POSIX)
-  logger.h / logger.c  Level-filtering logger
-  log_write.h/.c       stdout log sink
+  crsf.h / crsf.c         CRSF protocol: CRC8/DVB-S2, frame build/parse
+  crsf_telemetry.h/.c     Telemetry frame decoders (GPS, battery, link..)
+  crsf_stream.h/.c        Incremental streaming frame reader
+  crsf_param.h/.c         Parameter protocol (read/write/set_power)
+  serial.h / serial.c     Serial port abstraction (Win/POSIX)
+  logger.h / logger.c     Level-filtering logger
+  log_write.h/.c          stdout log sink
 tools/
-  telemetry.c          Telemetry read tool
+  telemetry.c             Telemetry read tool
 tests/
-  test_crsf.c          CRSF CRC, parse, build tests
-  test_serial.c        Serial open/close/stub tests
-  test_logger.c        Logger level-filtering tests
+  test_crsf.c             CRSF CRC, parse, build tests
+  test_serial.c           Serial open/close/stub tests
+  test_logger.c           Logger level-filtering tests
 deps/
-  FindUnity.cmake      Fetches Unity v2.6.1 via ZIP
-  FindCMock.cmake      Fetches CMock v2.6.0 via ZIP
+  FindUnity.cmake         Fetches Unity v2.6.1 via ZIP
+  FindCMock.cmake         Fetches CMock v2.6.0 via ZIP
 ```
 
 ## CRSF Protocol
@@ -94,22 +97,20 @@ deps/
 CRSF (Crossfire Serial Protocol) is the serial protocol used by ELRS for
 communication between ground station and TX/RX modules.
 
-### Frame format
+### Frame format (ELRS USB CRSF)
 
 ```
-+------+------+------+------+-------+-------+
-| 0xC8 | dest | src  | type | size  | ...   |  CRC  |
-+------+------+------+------+-------+-------+
-  1 byte  1B     1B     1B     1B     N B     1 byte
++--------+----------+--------+----------+-----+
+| addr   | length   | type   | payload  | CRC |
++--------+----------+--------+----------+-----+
+  1 byte   1 byte    1 byte    N bytes   1B
 ```
 
-- **Header:** Always `0xC8`
-- **Destination:** Target device address
-- **Source:** Sender device address
-- **Type:** Frame type (heartbeat, RC channels, telemetry, etc.)
-- **Size:** Payload length in bytes
+- **Address:** Frame sync byte (`0xC8` for host, `0xEE` for module, etc.)
+- **Length:** Total bytes after this field (type + payload + CRC)
+- **Type:** Frame type (RC channels, telemetry, parameter, etc.)
 - **Payload:** Frame-specific data
-- **CRC:** CRC8-CCITT over dest+src+type+size+payload
+- **CRC:** CRC8/DVB-S2 (poly `0xD5`) over type + payload
 
 ### Common device addresses
 
@@ -118,17 +119,23 @@ communication between ground station and TX/RX modules.
 | 0x00    | FC Broadcast                  |
 | 0x10    | Flight Controller             |
 | 0x80    | TBS Ground Station            |
-| 0xEA    | Custom Module                 |
-| 0xDD    | RC Device                     |
+| 0xEA    | Custom Module (Radio)         |
+| 0xEE    | ELRS TX Module                |
+| 0xEF    | ELRS Lua (host script)        |
 
 ### Common frame types
 
 | Type   | Name                        |
 | ------ | --------------------------- |
 | 0x01   | RC Channels Packed          |
-| 0x02   | Packet Link Telemetry       |
-| 0x03   | Heartbeat                   |
-| 0x08   | Device Info                 |
-| 0x09   | Parameter List              |
-| 0x17   | MSP Read                    |
-| 0x18   | MSP Write                   |
+| 0x02   | GPS                         |
+| 0x08   | Battery                     |
+| 0x0B   | Heartbeat                   |
+| 0x14   | Link Stats                  |
+| 0x16   | RC Channels                 |
+| 0x28   | Device Ping                 |
+| 0x29   | Device Info                 |
+| 0x2B   | Parameter Entry             |
+| 0x2C   | Parameter Read              |
+| 0x2D   | Parameter Write             |
+| 0x2E   | ELRS Status                 |