feat(parser_app): verify gateway-signed payment markers

[pepe-anchor]

Why

Enclave-side half of the x402 trust pair. parser_app has to check that a VerifiedPaymentMarker was signed by the pinned gateway key and is bound to this exact request, otherwise a paid parse could be replayed against a different transaction.

This is also where a wire collision gets fixed. ParseRequest field 4 shipped on main as include_intermediate_output (#414) while payment_marker was still sitting on the unmerged x402 branch claiming the same number. Renumbering now is free. Renumbering after the field is deployed is not.

What

• ParseRequest.payment_marker takes field 5, leaving include_intermediate_output = 4 untouched.
• host_primitives::payment_marker: the VerifiedPaymentMarker / SignedVerifiedPaymentMarker types, VPM_VERSION, and request_hash. Borsh-encoded, because the encoding has to match wire-for-wire between the gateway that signs and the enclave that verifies.
• parser_app::payment_verify: PaymentPolicy (Disabled or Required) and the signature plus request-binding check, run as the first thing parse() does.
• parse() gains a &PaymentPolicy parameter. Every existing caller passes Disabled, so nothing changes behavior in this PR.

PaymentPolicy::from_env from the branch was dropped on purpose: this repo cannot set env vars in tests (edition 2024 plus forbid(unsafe) bans std::env::set_var), and the real binaries take config from CLI args. from_hex is the only constructor.

Test evidence

cargo test -p host_primitives -p parser_app -p parser_grpc_server  -> all pass
make -C src test   -> full workspace green, every "test result" line ok, 0 failed
cargo fmt -- --check -> clean
make -C src lint   -> exit 0, no warnings
make -C src generated (re-run) -> no further diff, codegen is deterministic

Callers updated: parser_app::service, parser_grpc_server::main (both now pass &PaymentPolicy::Disabled), and parser_gateway::main picks up the new struct-literal field.

Rollback

Revert the commit and re-run make -C src generated. The proto field is additive and unused by any deployed caller, so nothing on the wire depends on it yet.

Linear

PRS-581

Stack position: based on main, independent of the pivot PRs. #414.

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