examples/sila: a Nano served as a SiLA 2 device, one field of each kind

[neu-rah]

A new example: one machine of eight roles, served as SiLA 2 features by a Python gateway, from a Nano or from a host build with simulated endpoints.

• Roles: switch, bounded level, discrete set, action, labelled select, analog in volts, text, and a command in presented units (a percentage, rounded to the nearest raw value, the device's actual value reported back).
• `gateway.py` knows only the kinds in its table (`KINDS`) and the unit symbols in `UNITS` (`V`, `%`); anything else, a tuned role, or a stray description line is refused by name at start. Feature definitions are generated at start, never committed. The server UUID is derived from the originator (`io.github.internetofpins`) and the machine hash.
• The six kinds that are not in `role/kinds.h` (discrete, action, select, analog, text, scaled) are in `examples/sila/src/kinds.h`; they move into the library when a second machine needs them. The gateway stays in the example until a second example needs it.
• `README.md`: wiring, build, flash, serve, run without a board (`--sim`), the field-to-SiLA-type table, the rules a user meets, expected output from a real Nano run, what is not covered. `README.md` (root) lists the example; `docs/role.md` links it.

Verified on a Nano (ATmega328P, avr-gcc 7.3.0): 8910 B flash / 345 B RAM; the SiLA checks (constraints, subscriptions, refusals, rounding edge cases, concurrency) pass with a separate client implementation (`unitelabs-sila`) and the XSD of `sila_base`; the host build and README steps work from a clean clone.

Two commits on top of `main` (the `value`/`scale`/`unit` lines and `%` landed in #24).

🤖 Generated with Claude Code