Add firmware internals section documenting RE work

New reference pages covering the V2.4a reverse engineering:
- Overview with architecture diagram and memory layout
- Command dispatch: XOR pattern, two dispatchers, compound commands
- Memory map: RAM variables, EEPROM layout, SFR definitions
- I2C & synthesizer: RDA1846S communication, frequency programming
- Source files: links to annotated assembly, C port, Python tools

These document the 19,053-line annotated assembly and 12,020-line
readable C port produced by Phases 1-3 of the RE project.

astro.config.mjs

@@ -65,6 +65,17 @@ export default defineConfig({
 						{ label: 'Connectors', slug: 'reference/connectors' },
 						{ label: 'Schematics', slug: 'reference/schematics' },
 						{ label: 'Hardware Revisions', slug: 'reference/hardware-revisions' },
+						{
+							label: 'Firmware Internals',
+							collapsed: true,
+							items: [
+								{ label: 'Overview', slug: 'reference/firmware' },
+								{ label: 'Command Dispatch', slug: 'reference/firmware/command-dispatch' },
+								{ label: 'Memory Map', slug: 'reference/firmware/memory-map' },
+								{ label: 'I2C & Synthesizer', slug: 'reference/firmware/i2c-synthesizer' },
+								{ label: 'Source Files', slug: 'reference/firmware/source-files' },
+							],
+						},
 					],
 				},
 				{