feat(kerykeion): crate scaffold with mesh types, config, and protobuf codegen

[forkwright]

Summary

• New kerykeion crate for Meshtastic mesh networking
• Protobuf code generation from 8 vendored Meshtastic .proto files via prost-build
• Core types: NodeNum, PacketId, ChannelIndex newtypes with validation
• MeshConfig with TOML deserialization (serial/TCP/BLE connections, channel PSKs)
• MeshConnection async trait for transport abstraction (implementations in P2-02)
• NodeDb for in-memory node tracking with insert/get/remove/iter
• Collector trait + MeshCollector stub
• snafu Error enum with 14 variants, all with implicit Location tracking
• New workspace deps: prost, tokio-serial, btleplug, aes, ctr, x25519-dalek, petgraph, mdns-sd, rusb, tokio-util, bytes, proptest, nix, uuid, prost-types

Test plan

• cargo fmt --all -- --check passes
• cargo clippy --workspace --all-targets -- -D warnings passes
• cargo test --workspace — 28 kerykeion tests + 385 workspace total, 0 failures
• cargo test --workspace --doc passes
• Protobuf encode/decode roundtrip (MeshPacket, ToRadio, FromRadio)
• Config TOML deserialization (MeshConfig from inline TOML string)
• NodeDb CRUD: insert, get, remove, iter, replace
• ChannelIndex out-of-range validation
• snafu error chain formatting and display
• Frame constants (FRAME_MAGIC, MAX_PACKET_SIZE, BROADCAST_ADDR)

Observations

Observation 1 — Debt (src/collector.rs:1): koinon::Collector does not exist as a trait. The Collector trait is defined locally in kerykeion::collector as a placeholder. Before P2-05 (gateway bridge), either koinon should be extended with a Collector trait (preferred — shared contract across all crates) or the local definition should be moved to a dedicated akroasis-core crate.

Observation 2 — Idea (crates/kerykeion/proto/meshtastic/): Vendored proto files are pinned to a snapshot derived from Meshtastic firmware knowledge as of 2026-03-18. A proto/VERSION file noting the upstream protobufs release tag (e.g. v2.5.18) would make upgrade diffs clearer and help CI detect drift.

Observation 3 — Missing test (src/config.rs): The ChannelPsk with non-empty PSK bytes (16-byte AES-128 key) has no test. The existing config test uses an empty PSK slice. A roundtrip test with a 16-byte PSK would catch accidental base64/binary encoding regressions when the TOML layer changes.

Observation 4 — Doc gap (src/connection.rs): MeshConnection has no crate-level example showing how a concrete implementation is expected to be wired up. The first implementor (P2-02 serial) will need to discover the Send + Sync constraint on the associated future types through the compiler rather than documentation.

🤖 Generated with Claude Code

[github-actions]

⚠️ Large PR detected — 19 files, 1697 lines changed.

Consider splitting into smaller PRs for easier review. Not a blocker, just a signal.