Phase 2: serial + RIO protocol core (RioJoy.Core) with unit tests

Port the RIO wire protocol from legacy/riovjoy2.cpp into testable C#:

- Protocol/: command + length table, 7-bit checksum, packet builder, and a
  streaming receive-side framing state machine (PacketParser) that mirrors the
  legacy ReadCommBlock framing/resync (high-bit-mid-packet abort). Typed RIO->PC
  decodes: AnalogReport (14-bit sign-extend), VersionInfo, CheckStatus; lamp-state
  composition.
- Serial/: RioSerialLink drives an async receive loop with ACK/NAK reply policy
  (legacy force-accept vs. opt-in VerifyInboundChecksum), the analog poll timer,
  and the >5s reset-recovery watchdog. IRioTransport abstracts the COM port; the
  SerialPort-backed transport does 9600 8N1 + DTR reset pulse, and acquire/release
  is just create/dispose (foundation for native-game serial yield).
- tests/RioJoy.Core.Tests: 54 xUnit tests covering checksum, framing/resync,
  builder round-trips, analog sign-extension + sentinel rejection, lamp combos,
  and the read loop driven against an in-memory fake transport.

Hardware verification (version/check/analog against a cabinet) remains; it can't
be done off-device.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

README.md

@@ -16,6 +16,7 @@ Red Planet — talk to the RIO directly and do not use this app.)
 |------|----------|
 | [`src/RioJoy.Core`](src/RioJoy.Core/) | Protocol, profile model, input mapper, HID feeder (class library) |
 | [`src/RioJoy.Tray`](src/RioJoy.Tray/) | Background tray application |
+| [`tests/RioJoy.Core.Tests`](tests/RioJoy.Core.Tests/) | xUnit tests for the protocol core |
 | [`driver/`](driver/) | `RioGamepad` virtual HID driver (KMDF + VHF) — replaces vJoy |
 | [`docs/PLAN.md`](docs/PLAN.md) | Full modernization plan (7 phases) |
 | [`docs/PROTOCOL.md`](docs/PROTOCOL.md) | RIO wire format + `iRIO` input-map reference |
@@ -29,8 +30,10 @@ Requires the **.NET 8 SDK** and Windows. The driver builds separately with the
 
 ```sh
 dotnet build RioJoy.sln -c Release
+dotnet test RioJoy.sln
 ```
 
 ## Status
 
-Phase 0 (scaffold + plan). See [`docs/PLAN.md`](docs/PLAN.md) for what's next.
+Phase 2 (serial + RIO protocol core) is code-complete and unit-tested; hardware
+verification is pending. See [`docs/PLAN.md`](docs/PLAN.md) for the full roadmap.