feat: pluggable signature schemes — generic chassis, ECDSA scheme, certificate taps

[bagelface]

Summary

Stacked on #184. Makes the aggregation actor chassis generic over the certificate scheme and p2p identity, adds an ECDSA (secp256k1) scheme alongside BN254, and exposes the seams an application-side scheme needs to plug in — deliberately without shipping scheme-specific operational machinery in the library.

Informed by gas-killer/service#312 and gas-killer/service#314 (aggregate Schnorr): the library's job is to allow a variety of schemes, not to guarantee every scheme's tooling. Schemes that are pure single-shot certificate::Scheme implementations ship in core as batteries; anything needing infrastructure beside the engine (nonce registries, spend journals, completion rounds) stays in the application, consuming the generic hooks added here.

Changes

• Chassis genericization: CertReporter<E, S> / CertifiedHeight<S> / NodeReporter<T, S> over S: certificate::Scheme; new CertIndex trait (get_tip/get) decoupling the sequencer from the concrete reporter — Sequencer<T, P, S, R: CertIndex, TS>; identity-generic ingest/TipReports<P>/recipients. Matches the trait shapes the service converged on, so its migration to the library stays mechanical.
• ECDSA scheme (core::ecdsa): operator's Ethereum key = p2p identity (20-byte address) = signing key; canonical low-s 65-byte signatures enforced at decode; EcdsaCertificate = signer bitmap + per-signer signatures in participant order. EcdsaSubmitter + EcdsaVerificationHandler seam hand (operators[], signatures[]) to the application (e.g. ERC-1271 stake-registry verification). No new bindings needed.
• Certificate taps: node-side CertificateObservation<S> opt-in channel (NodeReporter::with_certificate_tap), replay-deduplicated, carrying the opaque S::Certificate — the hook an application-side actor (e.g. a Schnorr completion round) consumes; router-side observation was already generic via CertifiedHeight<S>.
• README: documents the pluggable-scheme architecture and the app-side extension path.

BN254 remains fully supported and is the counter example's scheme — the example needed only type-annotation changes, demonstrating the genericization is behavior-preserving.

Validation

cargo check --workspace --all-targets, 83 tests (workspace, all passing), clippy --workspace --all-targets --all-features -- -D warnings, cargo fmt --check — all clean. New test coverage: full EcdsaScheme suite (quorum boundaries, tampered certificates, codec, ordering), tap delivery/replay-dedupe/dropped-receiver tests on the deterministic runtime.