feat: hardware-verifiable x402 payments

[BitHighlander]

Summary:

• validate official Solana exact PaymentRequirements before passing payment metadata to firmware
• route zero-LUT v0 transactions through SolanaSignTx instead of the blind message-signing fallback
• recognize official EIP-3009 exact-payment typed data on EVM
• document the REST x402 request and pin canonical device-protocol and hdwallet staging merge SHAs

Validated locally:

• 15 focused EVM/Solana x402 and signing-route tests pass
• production Vault bundle succeeds against the exact canonical submodule pins

Release gate:

• depends on the fork firmware PR and real KeepKey hardware validation
• keep this draft until the firmware artifact passes on-device

[BitHighlander]

Superseded by #403, which cherry-picks b373dfc7 onto current develop.

Not carried over: chore: pin canonical x402 dependencies. It moves modules/device-protocol to 5a8e2702 and modules/hdwallet to 25310f2c, and develop's current pins already contain both — merging it would have walked two tier-1 submodules backwards days before a release, for no gain. #403 takes the code and leaves the pins untouched.

Two fixes were needed on top, both to hold the typecheck gate at its 636 baseline (this branch pushed it to 637):

• solana-x402.ts imported from node:crypto, which does not resolve under this tsconfig
• neither x402 test file was in make test-unit, so they would never have run in CI

Closing this one.