ic402 v2.15.0

Minor — EvmSigner accepts a derivation path. Additive: every current caller compiles
and behaves exactly as in 2.14.0, STABLE_SCHEMA_VERSION stays 1, and no stable state is
touched.

Threshold Schnorr has taken a path per call since 2.14.0; EvmSigner took none at all — it
passed derivation_path = [] at all four of its management-canister sites, so a deployment
labelling its keys could not label the EVM one. It now takes a path per instance, not
per call, because an EVM address is published to payers and must never move.

Added

• EvmSignerAt(ecdsaKeyName, derivationPath) → { #ok : Signer; #err : Text }. Validates
  the path with SchnorrSigner.validateDerivationPath — the same
  MAX_DERIVATION_PATH_ELEMENTS (254) and MAX_DERIVATION_PATH_BYTES (8_192) bounds,
  reused rather than restated so the two signers cannot drift apart on what a valid path is.
  It returns #err; it never traps.
• EvmSigner(ecdsaKeyName) is unchanged — same name, same one-argument call, same empty
  path, same addresses. EvmSigner also remains a type, so existing annotations still
  resolve.
• Example endpoints evmAddressAt / signTypedDataAt showing the labelled-path form.

Notes

• The path is fixed at construction. No setter, no per-call override: an address derived
  under a path gets published, and letting it move afterwards would strand funds sent to the
  old one. Choose before publishing an address.
• The empty path is a default, not a decision — it is what the library did before paths
  existed, kept so no caller has to change. A deployment running more than one purpose off
  one key name should give each its own labelled path.
• Why factories rather than a second constructor. Motoko has neither constructor
  overloading (two classes of one name is M0051) nor a constructor that can return a
  Result (M0134). Both forms are therefore functions over one class, Signer, which is
  what keeps the one-argument spelling working while still allowing a refusal.
• The public-key/address cache is per-instance class state, so two signers on different paths
  cannot share an entry — no cache-key change was needed.

Testing

• 6 Motoko unit tests over construction and validation, including a drift test asserting
  EvmSignerAt accepts and refuses at exactly the counts SchnorrSigner does.
• 9 replica-backed tests (test/evmsigner-path.test.ts): two paths on one key name derive
  different addresses; each signature recovers independently (via @noble/curves, from
  the returned digest and r/s/v — not the canister's own signer field) to that path's own
  address; the one-argument form and the explicit [] form derive the same address; and
  over-limit paths are refused in-library.
• Mutation-verified: reverting the four sites to derivation_path = [] fails exactly the
  three path-dependence tests, and replacing the shared bound with a private looser one fails
  the drift test.