Add Phase 1 experimenter tools: MCP server, H21cm, beacon logger, arc survey

Four new tools transforming the SkyWalker-1 from satellite TV receiver into
a general-purpose RF observatory:

- skywalker-mcp: FastMCP server exposing 20 tools, 4 resources, 2 prompts.
  Thread-safe DeviceBridge with motor safety (continuous drive opt-in),
  input validation on all frequency/symbol rate/step parameters,
  try/finally on TS capture, path traversal sanitization, and reduced
  lock scope so emergency motor halt isn't blocked during long surveys.

- h21cm.py: Hydrogen 21 cm drift-scan radiometer at 1420.405 MHz with
  Doppler velocity calculation, control band comparison, and CSV output.

- beacon_logger.py: Long-term Ku-band beacon SNR/AGC logger with auto-relock,
  dual CSV/JSONL output, signal handlers, and systemd unit generation.

- arc_survey.py: Multi-satellite orbital arc census with USALS motor control,
  per-slot catalog persistence, resume support, and defensive motor halt
  on all error/interrupt paths.

Documentation: experimenter's roadmap guide + 4 tool reference pages (48 pages total).

site/src/content/docs/tools/mcp-server.mdx

@@ -0,0 +1,112 @@
+---
+title: MCP Server
+description: Model Context Protocol server that exposes the SkyWalker-1 hardware API to LLMs for autonomous RF exploration.
+---
+
+import { Aside, Steps } from '@astrojs/starlight/components';
+
+The `skywalker-mcp` package wraps the entire SkyWalker-1 Python API as an MCP (Model Context Protocol)
+server, making every hardware function accessible to LLMs. This enables natural-language signal analysis,
+autonomous RF exploration, and scheduled observation campaigns.
+
+## Installation
+
+```bash
+cd mcp/skywalker-mcp
+uv sync
+```
+
+## Running
+
+```bash
+# Local development
+uv run --directory mcp/skywalker-mcp skywalker-mcp
+
+# Add to Claude Code
+claude mcp add skywalker-mcp -- uv run --directory mcp/skywalker-mcp skywalker-mcp
+```
+
+## Tools (20)
+
+### Device Status
+| Tool | Description |
+|---|---|
+| `get_device_status` | Firmware version, config bits, USB speed, serial, last error |
+| `get_signal_quality` | SNR, AGC, power, lock status |
+| `get_stream_diagnostics` | Poll count, overflows, sync loss |
+
+### Spectrum & Tuning
+| Tool | Description |
+|---|---|
+| `sweep_spectrum` | Full-band power measurement with peak detection |
+| `tune_frequency` | Tune to specific freq/modulation/FEC, read signal |
+| `run_blind_scan` | Symbol rate sweep at single frequency |
+
+### Survey & Catalog
+| Tool | Description |
+|---|---|
+| `run_carrier_survey` | Six-stage pipeline: sweep → peaks → blind → TS → catalog |
+| `compare_surveys` | Diff two saved catalogs for changes |
+| `list_surveys` | List saved survey files with metadata |
+
+### Dish Motor
+| Tool | Description |
+|---|---|
+| `move_dish` | Halt, east, west, goto slot, USALS GotoX (continuous drive requires explicit opt-in) |
+| `jog_dish` | Small steps (1-30) + signal quality readback |
+| `store_position` | Save current position to memory slot |
+
+### LNB & I2C
+| Tool | Description |
+|---|---|
+| `set_lnb_config` | Voltage (13V/18V), 22 kHz tone, power off |
+| `scan_i2c_bus` | Enumerate all I2C devices |
+| `read_i2c_register` | Read single byte from I2C address |
+
+### Transport Stream & Identification
+| Tool | Description |
+|---|---|
+| `capture_transport_stream` | Capture + parse PAT/PMT/SDT for service names |
+| `identify_frequency` | Look up frequency against allocation tables |
+
+## Resources
+
+| URI | Description |
+|---|---|
+| `skywalker://status` | Live device state (firmware, config, signal) |
+| `skywalker://catalog/latest` | Most recent survey catalog as JSON |
+| `skywalker://allocations/lband` | L-band frequency allocation table |
+| `skywalker://modulations` | Supported modulations and FEC rates |
+
+## Prompts
+
+| Prompt | Description |
+|---|---|
+| `explore_rf_environment` | Strategy for autonomous RF discovery |
+| `hydrogen_line_observation` | Guided 21 cm observation procedure |
+
+## Architecture
+
+<Aside type="note" title="Thread safety">
+The BCM4500 demodulator cannot handle overlapping USB control transfers. The MCP server
+uses the same `DeviceBridge` pattern as the TUI — a `threading.RLock` serializes all
+hardware access. Async MCP handlers use `asyncio.to_thread()` to avoid blocking the
+event loop during USB I/O.
+</Aside>
+
+The server uses FastMCP's lifespan pattern: the USB device opens on server startup and
+closes on shutdown. All tools receive the device bridge through the lifespan context.
+
+## Testing
+
+```bash
+# Verify the server starts and can talk to hardware
+claude -p "What firmware version is loaded?" \
+    --mcp-config .mcp.json \
+    --allowedTools "mcp__skywalker-mcp__*"
+
+# Run a spectrum sweep via natural language
+claude -p "Sweep the full IF band and tell me what you find" \
+    --mcp-config .mcp.json \
+    --allowedTools "mcp__skywalker-mcp__*"
+```