Add USB-RS485 bridge and PC-side test suite

Makes end-to-end verification of the bus possible with a spare RP2040 and
no dedicated RS485 dongle.

bridge/ turns a second Pico into a transparent USB-to-RS485 bridge. It is
deliberately protocol-agnostic: bytes from USB go out on the pair, bytes
from the pair go back up USB, and it knows nothing about frames,
addresses or CRCs. A protocol-aware bridge could have a bug that happens
to agree with the node's own, which would defeat the point of using it as
a test instrument. It also needs no register-level code — it never has to
delimit a frame, and the one thing it does need, knowing that the final
stop bit has cleared the shifter before releasing DE, rp2040-hal exposes
as uart_is_busy().

tools/wiredsensor.py is an independent implementation of the wire format,
written from the specification in README.md rather than sharing code with
wiredsensor-core. Shared code would let a framing or CRC bug cancel out
and every test pass regardless. Cross-checked: it reproduces the
CRC-16/MODBUS catalogue vector and builds byte-identical frames to the
Rust side for all five documented examples.

Its `test` subcommand covers what host tests structurally cannot, all of
it timing-dependent: that malformed and mis-addressed frames leave the
node silent rather than colliding with whoever was actually addressed,
that the driver-enable turnaround leaves replies intact, and that no
state leaks between back-to-back frames.

Uses one wiring diagram for both boards, since the bridge mirrors the
node's GPIO0/1/2 assignment. Frames must fit one 64-byte USB packet, as
each host write becomes exactly one DE-bracketed transmission; every real
command is at most 21 bytes.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Cargo.lock

@@ -567,6 +567,12 @@ dependencies = [
  "syn 2.0.119",
 ]
 
+[[package]]
+name = "panic-halt"
+version = "1.0.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "a513e167849a384b7f9b746e517604398518590a9142f4846a32e3c2a4de7b11"
+
 [[package]]
 name = "panic-probe"
 version = "1.0.0"
@@ -1195,6 +1201,21 @@ dependencies = [
  "windows-link",
 ]
 
+[[package]]
+name = "wiredsensor-bridge"
+version = "0.1.0"
+dependencies = [
+ "cortex-m",
+ "cortex-m-rt",
+ "embedded-hal 1.0.0",
+ "fugit",
+ "panic-halt",
+ "rp2040-boot2",
+ "rp2040-hal",
+ "usb-device",
+ "usbd-serial",
+]
+
 [[package]]
 name = "wiredsensor-core"
 version = "0.1.0"