[wip] protocol spec

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+# protocol specs
+
+under construction!
+
+## client/robot
+
+the included wixel is used for bidirectional wireless serial data transfer. it
+has built-in error correction and other features that make sure data gets to
+the robot losslessly, so data loss is not accounted for.
+
+### important constants
+
+|property|value|unit|
+|-|-|-|
+|baud rate|9600|bit s⁻¹|
+|max byte timeout|5|ms|
+|byte order|big-endian|-|
+
+### commands
+
+each command consists of an opcode, and then a payload. each opcode defines
+logic to handle payload length, so certain commands might expect a fixed-length
+payload, a variable-length payload, or none at all.
+
+opcodes are picked sequentially, but bit 0 is reserved to indicate a transfer
+from robot to client. this means that the opcode for a sensor data request
+would be `0x14`, but the response opcode would be `0x15`. a c header is
+provided that contains consts of these opcodes for code readability.
+
+|code|name|implemented|directions|full name|
+|--:|---|:-:|:-:|---|
+|`0x00`|[BOMD](#bomd)|no|`r <=> c`|<u>b</u>ack<u>o</u>rder <u>m</u>o<u>d</u>ify
+|`0x00`|[CORD](#cord)|no|`r <=> c`|<u>co</u>o<u>rd</u>inate
+|`0x00`|[EXFL](#exfl)|no|`r <-- c`|<u>ex</u>ecution <u>fl</u>ow
+|`0x00`|[SRES](#sres)|no|`r <-- c`|<u>s</u>oft <u>res</u>et
+|`0x00`|[EXPT](#expt)|no|`r --> c`|<u>ex</u>ce<u>pt</u>ion
+|`0x00`|[SPED](#sped)|no|`r <-- c`|<u>spe</u>e<u>d</u>
+|`0x00`|[DIRC](#dirc)|no|`r <-- c`|<u>dir</u>ect <u>c</u>ontrol
+|`0x00`|[DISP](#disp)|no|`r <-- c`|<u>disp</u>lay control
+|`0x00`|[SENS](#sens)|no|`r <=> c`|<u>sens</u>or data
+|`0x00`|[MAPS](#maps)|no|`r <-- c`|<u>map</u> <u>s</u>end
+|`0x00`|[PING](#ping)|no|`r <=> c`|<u>ping</u>
+|`0x00`|[PLAY](#play)|no|`r <-- c`|<u>play</u> midi
+|`0x00`|[CLED](#cled)|no|`r <-- c`|<u>c</u>ontrol <u>led</u>s
+
+#### BOMD
+
+##### backorder append
+
+- length: 2 bytes
+- client to robot
+
+|type|description|
+|-:|-|
+|`uint8_t`|opcode (`0x00 + 0`)|
+|`uint8_t`|action|
+
+_action_ is a byte containing `0bXXXXXXYY` where X+1 is the amount of times
+action Y gets repeated. Y can be:
+- 0: move 1 tile forward
+- 1: move 1 tile backward
+- 2: turn right 90°
+- 3: turn left 90°
+
+examples:
+- move forward 4 tiles: `0b00001100` (`0x10`)
+- turn 180°: `0b00000110` (`0x06`) or `0b00000111` (`0x07`)
+
+##### backorder index
+
+- length: 9 bytes
+- robot to client
+
+|type|description|
+|-:|-|
+|`uint8_t`|opcode (`0x00 + 1`)|
+|`uint32_t`|backorder buffer length|
+|`uint32_t`|backorder buffer index|
+
+#### CORD
+
+##### set position
+
+- length: 6 bytes
+- client to robot
+
+|type|description|
+|-:|-|
+|`uint8_t`|opcode (`0x00 + 0`)|
+|`uint32_t`|position|
+|`uint8_t`|orientation|
+
+_position_ = y * width + x  
+_orientation_:
+- 0: west
+- 1: north
+- 2: east
+- 3: south
+
+##### get position
+
+- length: 6 bytes
+- robot to client
+
+|type|description|
+|-:|-|
+|`uint8_t`|opcode (`0x00 + 1`)|
+|`uint32_t`|position|
+|`uint8_t`|orientation|
+
+_position_ = y * width + x  
+_orientation_:
+- 0: west
+- 1: north
+- 2: east
+- 3: south
+
+#### EXFL
+
+##### set execution flow
+
+- length: 2 bytes
+- client to robot
+
+|type|description|
+|-:|-|
+|`uint8_t`|opcode (`0x00 + 0`)|
+|`uint8_t`|action|
+
+_action_ is:
+- 0: suspend all operation
+- 1: resume operation
+
+#### SRES
+
+##### soft reset
+
+- length: 1 byte
+- client to robot
+
+|type|description|
+|-:|-|
+|`uint8_t`|opcode (`0x00 + 0`)|
+
+#### EXPT
+#### SPED
+#### DIRC
+#### DISP
+DCLR display clear  
+DPOS display cursor position  
+DWSR display write string  
+DWBM display write bitmap
+#### SENS
+#### MAPS
+#### PING
+#### PLAY
+#### CLED
+
+
+## client/websocket
+
+TBD
+