Add RF test bench tool for CW injection tests with NanoVNA + HMC472A

New tool (tools/rf_testbench.py) automates five test sequences using a
NanoVNA as a CW source and HMC472A digital attenuator (0-31.5 dB, 0.5 dB
steps via REST API) to characterize the SkyWalker-1 receiver:

- AGC linearity mapping across 64 attenuation steps
- IF band flatness sweep (950-1500 MHz)
- Frequency accuracy via peak detection
- Minimum detectable signal search
- BPSK mode 9 CW probe (Viterbi rate 1/2 K=7)

Includes SKYWALKER_MOCK=1 mode, path-loss calibration from NanoVNA S21
sweeps, and safe-state cleanup (attenuator to max on exit, LNB power
never enabled in direct-input mode).

Also adds Applications & Use Cases guide, RF Test Bench docs page, fixes
h21cm cable loss (was 3x too high), and updates sidebar.

site/astro.config.mjs

@@ -122,6 +122,7 @@ export default defineConfig({
             { label: 'TS Analyzer', slug: 'tools/ts-analyzer' },
             { label: 'Spectrum Analysis', slug: 'tools/spectrum-analysis' },
             { label: 'Hydrogen 21 cm', slug: 'tools/h21cm' },
+            { label: 'RF Test Bench', slug: 'tools/rf-testbench' },
             { label: 'Beacon Logger', slug: 'tools/beacon-logger' },
             { label: 'Arc Survey', slug: 'tools/arc-survey' },
             { label: 'MCP Server', slug: 'tools/mcp-server' },
@@ -131,6 +132,7 @@ export default defineConfig({
         {
           label: 'Guides',
           items: [
+            { label: 'Applications & Use Cases', slug: 'guides/applications' },
             { label: 'QO-100 DATV Reception', slug: 'guides/qo100-datv' },
             { label: "Experimenter's Roadmap", slug: 'guides/experimenter-roadmap' },
           ],