Refactor peripheral/bus/phy APIs (prep for pico/PIO)

- Refactor PHY to be higher-level with a field focus (controller/peripheral address,
  control byte, etc)
  - Phy layer is now device address aware (ACK's automatically) (Hardens
    previously ~implicit design to only support emulating one AVCLAN
    unit per mockingboard build)
- Redesign `phy_active` => `phy_frame_pending` (to allow
  asynchronous/buffered frame reads on pico) and embed within
  Peripheral::read
- Add `deafen` function: like mute, but also ignore incoming messages (i.e.
  don't receive/process at all). Makes `Bus::is_active` always return
  `false`.
- Add debug bus driving functions

Assisted-by: Claude Code (Opus 5)

src/avclan/peripheral.hpp

@@ -40,7 +40,7 @@ public:
                 "update the sentinel value in avclan.h");
 
   Peripheral(Bus &bus, uint16_t address) : bus{bus}, address_{address} {
-    bus.init();
+    bus.init(address);
     (std::get<Devs>(devices_).init(), ...);
   }
 
@@ -59,7 +59,10 @@ public:
 
   expected<std::unique_ptr<Frame>, Error::Read>
   read(Frame::Print print = Frame::Print{}) {
-    return bus.read(address_, print);
+    if (!bus.is_active())
+      return unexpected{detail::Error::Read::NO_FRAME};
+
+    return bus.read(print);
   };
 
   expected<std::unique_ptr<Frame>, detail::SendError>