Add MCP server for AI-assisted device interaction

Host-side MCP (Model Context Protocol) server in C that provides
tools for AI assistants to interact with MIOS devices over USB.

Tools:
- configure: set USB VID/PID for device discovery
- cli: send CLI commands via USB MCP interface
- read_memory: read raw device memory (max 32 bytes)
- flash_dfu: flash firmware via USB DFU with auto-detach
- flash_stlink: flash via OpenOCD TCL interface
- sigcapture_list: list recent signal captures from background thread
- sigcapture_save: export captures to CSV for analysis

Also refactors support/dfu/dfu.c to reuse shared code from host/dfu.c
which handles DFU runtime detection, detach, and flashing.

Build with: make -C host/mcp
Configure in ~/.claude.json as an MCP server.
See docs/mcp.md for setup and usage details.

Author: andoma

docs/mcp.md

@@ -0,0 +1,150 @@
+# MIOS MCP Server
+
+MCP (Model Context Protocol) server that allows AI assistants to interact
+with MIOS devices over USB. Supports CLI command execution, memory reads,
+firmware flashing (DFU and ST-Link), and signal capture.
+
+## Device-side setup
+
+Add the USB interfaces to your platform's interface queue:
+
+```c
+#include <usb/usb.h>
+
+struct usb_interface_queue q;
+STAILQ_INIT(&q);
+
+usb_cdc_create_shell(&q);            // Optional: serial console
+usb_dfu_runtime_create(&q);          // Optional: in-app DFU detach
+usb_mcp_create(&q, 0x01);            // MCP command interface (subclass 0x01)
+
+your_platform_usb_create(vid, pid, manufacturer, product, &q);
+```
+
+The MCP command interface uses USB vendor class (0xFF) with subclass
+0x01. Sigcapture uses subclass 192. These are separate USB interfaces.
+
+Rebuild and flash your firmware.
+
+## Building the MCP server
+
+```
+make -C host/mcp
+```
+
+Produces `host/mcp/mios-mcp`. Requires `libusb-1.0` development
+headers (`apt install libusb-1.0-0-dev` on Debian/Ubuntu).
+
+## Claude Code integration
+
+Add to `~/.claude.json`:
+
+```json
+{
+  "mcpServers": {
+    "mios": {
+      "command": "/path/to/mios/host/mcp/mios-mcp"
+    }
+  }
+}
+```
+
+Restart Claude Code. The following tools become available:
+
+### configure
+
+Set the USB VID/PID used to find the MIOS device. Affects all
+subsequent tool calls. Default VID is 0x6666, PID is 0 (match any).
+
+```
+configure(vid: 0x1234, pid: 0x5678)
+```
+
+### cli
+
+Send a CLI command to the device via the USB MCP interface and get
+the text output back.
+
+```
+cli(command: "uptime")
+cli(command: "i2c scan 0", timeout_ms: 10000)
+```
+
+### read_memory
+
+Read raw memory from the device (max 32 bytes per call). Returns
+a hex dump.
+
+```
+read_memory(address: 0x08000000, length: 32)
+```
+
+### flash_dfu
+
+Flash firmware via USB DFU. Automatically handles DFU Runtime detach
+if the device is running (requires `usb_dfu_runtime_create` on device).
+
+```
+flash_dfu(elf_path: "/path/to/build.elf")
+flash_dfu(elf_path: "/path/to/build.elf", force: true)
+```
+
+### flash_stlink
+
+Flash firmware via a running OpenOCD instance (TCL interface, default
+port 6666). Auto-detects flash vs RAM targets from the ELF load address.
+
+```
+flash_stlink(elf_path: "/path/to/build.elf")
+flash_stlink(elf_path: "/path/to/build.elf", host: "127.0.0.1", port: 6666)
+```
+
+### sigcapture_list
+
+List recent signal captures stored in memory. A background thread
+continuously receives captures from the device and stores up to 16
+in a ring buffer. Returns metadata only (no sample data).
+
+```
+sigcapture_list()
+```
+
+### sigcapture_save
+
+Export a capture to CSV for analysis. The CSV has a `time_s` column
+(relative to trigger) followed by one column per channel with scaled
+values. Use the capture ID from `sigcapture_list`.
+
+```
+sigcapture_save(id: 42, path: "/tmp/capture.csv")
+```
+
+The CSV can be loaded with Python/pandas/matplotlib for analysis.
+
+## USB MCP command protocol
+
+The MCP command interface (`usb_mcp_create`, subclass 0x01) uses
+64-byte bulk transfers with a single-byte message type prefix:
+
+| Type | Direction    | Name             | Payload                          |
+|------|------------- |------------------|----------------------------------|
+| 0x01 | Host->Device | CLI Execute      | Command string (max 63 bytes)    |
+| 0x02 | Device->Host | CLI Response     | Output text (up to 63 bytes)     |
+| 0x03 | Device->Host | CLI Complete     | int32_t error code (LE)          |
+| 0x04 | Host->Device | Read Memory      | uint32_t addr + uint32_t length  |
+| 0x05 | Device->Host | Memory Response  | Raw data bytes                   |
+
+CLI flow: host sends 0x01, device sends N x 0x02 packets with output
+text, then 0x03 with the error code.
+
+## Sigcapture protocol
+
+Sigcapture uses a separate vendor-class USB interface (subclass 192)
+with a single bulk IN endpoint. The device pushes capture data after
+a trigger event. The MCP server receives this in a background thread.
+
+Packet dispatch by size:
+- 10 bytes: preamble (channels, depth, sample rate, trigger offset)
+- 20 bytes: channel descriptor (name, unit, scale)
+- 64 bytes: data (columnar int16_t samples)
+- 1 byte: trailer (0xfe, capture complete)