v0.6.2 — typed read-side block.actions[] parsing + signed-method smokes

Adds typed Action variant dispatch on the read side:
- BlockAction abstract base + 8 typed variants (Transfer / Mint / Burn /
  RegisterAccount / AddAccountKey / RevokeAccountKey /
  RegisterSessionKey / BridgeOut) + BlockActionUnknown fallback
- BlockActionParser.Parse / ParseArray (externally-tagged dispatch)
- BlockDetails.GetTypedActions() ergonomic getter
- JsonByteArrayConverter for the chain's number-array byte wire shape

Plus integration smokes for all 24 SignedXxxAsync methods using the
existing FakeSigner→4xx pattern.