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/astro.config.mjs

@@ -121,6 +121,10 @@ export default defineConfig({
             { label: 'Debugging', slug: 'tools/debugging' },
             { label: 'TS Analyzer', slug: 'tools/ts-analyzer' },
             { label: 'Spectrum Analysis', slug: 'tools/spectrum-analysis' },
+            { label: 'Hydrogen 21 cm', slug: 'tools/h21cm' },
+            { label: 'Beacon Logger', slug: 'tools/beacon-logger' },
+            { label: 'Arc Survey', slug: 'tools/arc-survey' },
+            { label: 'MCP Server', slug: 'tools/mcp-server' },
             { label: 'Safety Testing', slug: 'tools/safety-testing' },
           ],
         },
@@ -128,6 +132,7 @@ export default defineConfig({
           label: 'Guides',
           items: [
             { label: 'QO-100 DATV Reception', slug: 'guides/qo100-datv' },
+            { label: "Experimenter's Roadmap", slug: 'guides/experimenter-roadmap' },
           ],
         },
         {