Complete typed COM decoding

docs/host_com_rebuild.md

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+# Host COM Rebuild
+
+## Goal
+
+Keep the COM layer small, explicit, and easy to teach.
+
+The COM layer should:
+
+- define packet types and payload structures
+- build full wire frames
+- parse full wire frames
+- parse typed packet payloads
+
+The COM layer should not:
+
+- enumerate devices
+- open ports
+- own handshake state
+- own UI state
+
+## Current file roles
+
+### `Com_Cdc.h`
+
+Owns the packet model:
+
+- packet types
+- fixed payload lengths
+- payload structs
+
+### `Com_CdcEncode.h/.cpp`
+
+Owns the write direction:
+
+- checksum
+- full frame build
+- typed payload encode
+
+### `Com_CdcDecode.h/.cpp`
+
+Owns the read direction:
+
+- full frame parse
+- stream extraction
+- typed payload decode
+
+## Completed nodes
+
+### Node 1: legacy frame encode baseline
+
+Already present before this step:
+
+- build full frame
+- build all typed packets
+
+### Node 2: typed packet decode completion
+
+Files updated in this step:
+
+- `KeyBorad/KeyBorad/COM/Com_CdcDecode.h`
+- `KeyBorad/KeyBorad/COM/Com_CdcDecode.cpp`
+
+Design notes:
+
+- keep decode logic flat and explicit
+- one helper for little-endian reads
+- one helper for type + length validation
+- one decode function per packet type
+
+Implemented behavior:
+
+- parse frame header and checksum
+- extract frames from a byte stream
+- decode:
+  - `HelloReq`
+  - `HelloRsp`
+  - `Bitmap`
+  - `FunctionKeyEvent`
+  - `LedState`
+  - `TimeSync`
+  - `ThemeRgb`
+  - `Ack`
+  - `Error`
+
+## COM done criteria
+
+For the current project stage, COM is considered done when:
+
+1. every supported packet can be built
+2. every supported packet can be parsed
+3. stream extraction is stable
+4. higher layers no longer need to know byte offsets