Half-duplex becomes a compile-time feature with a shared-wire drive discipline

[aq1018]

Half-duplex worked point-to-point but couldn't sit on a shared (multi-drop) wire: the pin idled push-pull, driving against other talkers, and enabling TE before HDSEL latched the TX output LOW — clamping the bus from init until the first own transmission (measured on CH32V006).

Breaking: duplex selection moves from runtime config to compile time. The Duplex enum and UsartConfig::duplex are gone; single-wire operation is now the half-duplex cargo feature (rx_pull only exists without it). A bootloader binary serves one board, so there's no runtime switch worth paying for — full-duplex binaries are byte-identical to before, which matters on the CH32V003's 1920-byte system flash.

With the feature on, the wire follows a shared-bus drive discipline:

• usart::init raises HDSEL before TE, so the TX output idles at mark from the start.
• The single-wire pin parks open-drain and flips to push-pull only around writes — an idle node never drives the wire, so multiple devices can share one bus. The bus pull-up owns the idle level.

The transports guide, troubleshooting doc, and examples are updated, including the new hardware requirement: an external pull-up on the wire. CI gains a half-duplex clippy leg per family.