[austin] Improve runtime nonce readiness telemetry

[AustinKelsay]

Summary

• expand signer-owned peer readiness telemetry for browser/operator surfaces with per-method capabilities, response latency, and nonce inventory history
• add ping-time nonce reconciliation so peers report recognized nonce codes, stale inbound nonce inventory is pruned, and signing prefers freshly advertised peer nonces
• record responder-side PingServed, SignServed, and EcdhServed completions so hosts can log served requests consistently
• surface inbound request failures as operation failures with nonce-miss or operator-denied context
• align CI with the current workspace crate map and make devtools tests tolerate a missing external igloo-shell binary on CI

Why this belongs in bifrost-rs

• Readiness is signer state, not UI state: peer liveness, policy, nonce inventory, and request latency all live below the host/client layer.
• The stale-nonce failure mode cannot be safely fixed in Igloo UI code; only the nonce pool/signing device can decide which nonce codes are still valid for a peer.
• Host surfaces need canonical completions/failures from the signer so browser, Tokio, and future operator clients do not invent divergent lifecycle state.
• recognized_peer_nonce_codes and served-operation completions are runtime contracts, so they belong in the Rust core/bridge layer first.

Reviewer Notes

• PingPayloadWire now carries optional recognized_peer_nonce_codes; serde defaults keep older JSON payloads compatible, but direct Rust struct fixtures must set the field explicitly.
• EcdhServed mirrors the existing served-completion path for ping/sign so browser hosts can log responder-side ECDH activity.
• GitHub Actions are green on the current head fceca55.

Validation

• cargo fmt --all -- --check
• cargo clippy --workspace --all-targets --offline --no-deps -- -D warnings
• cargo check --workspace --offline
• cargo test -p bifrost-core -p bifrost-codec --offline
• cargo test -p bifrost-signer --test runtime_roundtrip --offline
• cargo test -p bifrost-signer
• cargo test -p bifrost-bridge-tokio
• cargo test -p bifrost-bridge-wasm
• cargo test --workspace --offline