drv/frame: add mari-compatible header for bare-radio apps

[geonnave]

Adds drv/frame.h — a 21-byte wire-header struct that's a byte-for-byte
mirror of Mari's mr_packet_header_t, plus the constants bare-radio DotBot
apps need to populate it: version = 3, type = DATA, next_proto = DOTBOT_APP, network_id = 0xD0B0, BLE access address 0xDB12DB12.

The intent: once bare-radio apps adopt this header, a bare-radio gateway
can wrap received frames as MARI_EDGE_DATA UART events and PyDotBot's
MarilibEdge adapter (which now dispatches by next_proto) handles them
with no special case. One frame vocabulary on the host whether the bytes
came from Mari TSCH or from a bare-radio app. Coexistence on air is
handled by the distinct access address — Mari's 0x12345678 vs bare's
0xDB12DB12 — so the two stacks ignore each other at the radio peripheral
filter, not in software.

Worth a second look:

• Numeric values are pinned against Mari's enum: type = 16 ↔
  MARI_PACKET_DATA, next_proto = 0x11 ↔ MARI_NEXT_PROTO_DOTBOT_APP,
  version = 3 ↔ MARI_PROTOCOL_VERSION. A _Static_assert(sizeof == 21) catches layout drift loudly.
• type and next_proto are stored as uint8_t, not the Mari enum
  types, so the layout doesn't depend on -fshort-enums ABI choices.
• network_id = 0xD0B0 needs to be reserved on the Crystalfree Mari
  network-ID registry before any real deployment uses it.

No consumer in this repo yet — DotBot-firmware bare-radio apps will
migrate from drv/tdma_client to this header in a follow-up PR. Build is
unchanged on every existing target; clang-format clean.