v0.6.1 — wallet-coverage signed endpoints + hex parity + README walkthrough

Adds the 4 signed endpoints chain shipped outside the 20-entry batch:
- SignedDepositFuelAsync (/deposit-fuel-signed)
- SignedNamespaceAdminAsync (/namespace-admin-signed)
- SignedPlayerManageAsync (/player-manage-signed)
- SignedPurchaseCapabilityWrappedAsync (/purchase-capability-signed)

Plus byte-for-byte hex parity tests against chain's bridge_golden.rs
for every borsh payload, and a README signed-mode walkthrough showing
the full sign → POST → response loop with a sample IActionSigner.

SignedEndpointRegistry widened from 20 to 24 entries.