v0.2.0 — Zod major bump

BREAKING: peer dep `zod` bumped 3.24 → 4.

Triggered by bridge-ui's same-day Valibot → Zod 4 migration and
their commitment to adopt @velocity/sdk/wallet across four call-sites.
SDK's Zod major now matches.

Wire shapes, public API, golden vectors all unchanged. 541/541 tests
pass cold against Zod 4. Schema source migrated to Zod 4 idiomatic
patterns (z.iso.datetime, z.email, z.url) for cross-team grep clarity.

The z.unknown() auto-optional quirk persists from Zod 3 to Zod 4;
existing SignedActionResponseSchema cast-workaround stays. Documented
in CHANGELOG.

Install:
  npm install git+https://gitlab.syncad.com/hive/velocity-js-sdk.git#v0.2.0

Not yet on npm. Same publish-when-real-consumer-validates gate as
v0.1.0 — bridge-ui's adoption is the validation event.