Add native USDT wallet valuation to read-only account facts

[Pigbibi]

Changes

• Add an optional USDT wallet valuation to the existing read-only account facts.
• Use one fixed signed wallet-balance GET with a 15-second timeout, no redirects and no retries.
• Sum each returned wallet row once with exact Decimal arithmetic; retain native Spot/Flexible Earn quantities when valuation is unavailable.
• Keep existing identity, source and no-order protections unchanged. The valuation covers provider-returned wallet rows and is separate from account NAV or USD totals.

Validation

• 144 focused synthetic tests passed, including the locked SDK's final request parameters, invalid responses, exact arithmetic and quantity fallback.
• The full local suite passed: 1,259 tests and 91 subtests. The SDK wire test runs in a clean child process to avoid other suites' module stubs.
• Ruff, compile checks and diff checks passed.
• Independent source review passed within this scope.

No real wallet request or production reader adoption is included in this source change.