feat: implement wire framing, RPC server/client, and CLI tool

[tarotene]

Summary

• Add COBS/rzCOBS framing layer to telepath-wire (framing.rs)
• Implement target-side RPC server with transport abstraction in telepath-firmware
• Implement host-side RPC client with frame size enforcement in telepath-host
• Add telepath-cli tool crate for host-side serial communication
• Update nrf52840-dk example with RTT transport integration
• Harden request/response validation: kind checks, payload size limits, split PayloadTooLarge variants
• Fix .gitignore to use **/target/ covering all subdirectories

Key Changes

Crate	What changed
telepath-wire	New framing module (COBS encode/decode, rzCOBS encode/decode, MAX_FRAME_SIZE)
telepath-firmware	RPC server (poll-based dispatch), transport module, payload/kind validation
telepath-host	RPC client (call_raw), frame size guard, response kind validation
telepath-cli	New crate — serial port CLI with ping command
nrf52840-dk	RTT transport, updated example wiring

Usage

Prerequisites

• probe-rs installed (cargo install probe-rs-tools)
• nRF52840-DK connected via USB
• thumbv7em-none-eabi target: rustup target add thumbv7em-none-eabi
• udev rules (Linux): USB access for SEGGER J-Link requires a udev rule

sudo tee /etc/udev/rules.d/99-probe-rs.rules <<'RULE'
# SEGGER J-Link
ATTR{idVendor}=="1366", MODE="0666", GROUP="plugdev"
RULE
sudo udevadm control --reload-rules && sudo udevadm trigger

1. Flash the firmware

cd examples/nrf52840-dk
cargo run --release

cargo run flashes the binary and opens an RTT session. The firmware
starts immediately; LED 1 blinks to indicate liveness.

Note: --allow-erase-all is included in the runner command
(see .cargo/config.toml). This is required on first flash when
APPROTECT (factory readback protection) is active; it has no effect
once the chip is unlocked.

2. Run the CLI

In a second terminal (firmware must remain running):

cd tools/telepath-cli
cargo build
./target/debug/telepath-cli ping

Expected output:

ping -> 0xDEADBEEF

To target a different chip:

telepath-cli --chip STM32F411RETx ping

3. Use telepath-host as a library

use telepath_host::TelepathClient;

// transport: anything implementing std::io::Read + std::io::Write
let mut client = TelepathClient::new(transport);

// Call CmdID 0x0001 (ping) with no arguments
let payload = client.call_raw(0x0001, &[])?;
let val: u32 = postcard::from_bytes(&payload)?;
println!("ping -> 0x{:08X}", val);  // ping -> 0xDEADBEEF

Test plan

• cargo test --workspace passes
• cargo clippy --workspace -- -D warnings clean
• cd examples/nrf52840-dk && cargo build --release succeeds
• cd tools/telepath-cli && cargo build succeeds

🤖 Generated with Claude Code

[Copilot]

Pull request overview

Updates ignore rules intended to prevent committed Rust build artifacts, but also introduces substantial new Telepath RTT/COBS protocol plumbing across host, firmware, examples, and a new CLI tool.

Changes:

• Update .gitignore to ignore target/ directories recursively.
• Add COBS framing utilities and integrate RTT-based transport + polling server on firmware, plus corresponding host framing in TelepathClient.
• Add telepath-cli and update the nRF52840-DK example to use RTT transport and a ping command.

Reviewed changes

Copilot reviewed 15 out of 16 changed files in this pull request and generated 8 comments.

Show a summary per file

File	Description
.gitignore	Switches to **/target/ ignore pattern.
Cargo.toml	Excludes tools/telepath-cli from the workspace.
tools/telepath-cli/Cargo.toml	Adds a standalone host-side CLI crate with probe-rs/clap deps.
tools/telepath-cli/src/main.rs	Implements RTT ping command using probe-rs and COBS framing.
crates/telepath-wire/src/lib.rs	Exposes new framing module.
crates/telepath-wire/src/framing.rs	Adds COBS encode/decode helpers and a FrameAccumulator with tests.
crates/telepath-wire/Cargo.toml	Adds cobs dependency (no_std).
crates/telepath-firmware/src/transport.rs	Introduces non-blocking byte-stream Transport trait.
crates/telepath-firmware/src/lib.rs	Adds RTT/COBS frame accumulation, request dispatch, and response framing in poll().
crates/telepath-firmware/Cargo.toml	Adds serde and postcard dependencies to support serialization in firmware crate.
crates/telepath-host/src/lib.rs	Implements COBS framing in TelepathClient::call_raw and adds tests.
crates/telepath-host/Cargo.toml	Adds cobs and a firmware dev-dependency for integration tests.
examples/nrf52840-dk/Cargo.toml	Switches RTT stack to rtt-target, adds postcard, changes panic impl.
examples/nrf52840-dk/.cargo/config.toml	Removes defmt linker script flag.
examples/nrf52840-dk/src/rtt_transport.rs	Adds RTT-backed Transport implementation for firmware server.
examples/nrf52840-dk/src/main.rs	Initializes RTT channels, registers ping command, and polls server in loop.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[tarotene]

Verification Checklist

Before merging, please run through the following two tiers.

Tier 1 — No hardware required (mirrors CI)

# Checkout this branch first
git fetch origin feat/claude-session-20260315-011025
git checkout feat/claude-session-20260315-011025

# A. Full workspace test suite — the loopback round-trips below MUST be green:
#    - crates/telepath-firmware  : poll_ping_roundtrip
#      (full COBS frame in → server.poll() → COBS frame out, 0xDEADBEEF asserted)
#    - crates/telepath-host      : server_ping_roundtrip
#      (validates wire compatibility across the host↔firmware boundary)
cargo test --workspace

# B. Lint / formatting
cargo fmt --all -- --check
cargo clippy --workspace -- -D warnings

# C. Firmware cross-compile
cd examples/nrf52840-dk && cargo build --release && cd -

# D. CLI build (workspace-excluded crate)
cd tools/telepath-cli && cargo build && cd -

All four steps pass in CI, but running them locally catches environment issues before merge.

---

Tier 2 — Real nRF52840-DK (optional but recommended)

Prerequisites

• rustup target add thumbv7em-none-eabi

• cargo install probe-rs-tools

• Linux udev rule (first-time setup): add a rule for the SEGGER J-Link onboard the DK:

  sudo tee /etc/udev/rules.d/99-probe-rs.rules <<'RULE'
  # SEGGER J-Link
  ATTR{idVendor}=="1366", MODE="0666", GROUP="plugdev"
  RULE
  sudo udevadm control --reload-rules && sudo udevadm trigger
  

Note on --allow-erase-all: the runner in examples/nrf52840-dk/.cargo/config.toml
already includes this flag. It is required on the first flash when factory APPROTECT is
active and has no effect once the chip is unlocked.

Steps

# 1. Flash firmware (probe-rs download — probe released after flash completes)
cd examples/nrf52840-dk && cargo run --release && cd -

#    Visual check: LED1 (P0.13) should blink at ~5 Hz, indicating the
#    server.poll() loop is alive.

# 2. One-shot ping via CLI (open a second terminal while firmware runs)
cd tools/telepath-cli && cargo run -- ping && cd -
#    Expected stdout : ping -> 0xDEADBEEF
#    Expected stderr : RTT debug log from firmware (forwarded from ch 0)

# 3. REPL smoke test
cd tools/telepath-cli && cargo run
#    Type "ping"  → ping -> 0xDEADBEEF
#    Type "help"  → lists commands
#    Type "quit"  → clean exit

Troubleshooting

Symptom	Cause	Fix
RTT down channel 1 not found	Reserved down channel 0 missing	Confirm commit a69f241 is in history
Failed to attach to RTT	APPROTECT not unlocked	Ensure --allow-erase-all in runner; power-cycle board; re-flash
ping times out after 5 s	server.poll() loop stalled	Check RTT ch 0 stderr for panic messages

---

Once Tier 1 passes (Tier 2 if hardware is available), this PR is ready for squash merge to preserve Conventional Commit history.