feat(payments): add Solana x402 and BTC rail readiness

[Svaag]

Summary

• Add x402 v2 exact SVM support for Solana mainnet alongside the existing EVM schemes.
• Gate every advertised x402 network against the facilitator supported-kind response; Solana additionally requires facilitator fee-payer metadata.
• Add explicit per-network receiver configuration and expose family-specific wallet metadata through quotes, the payment catalog, and MCP.
• Add a durable worker heartbeat, native-rail kill switch, provider health checks, pending-intent/scan-lag metrics, and fail-closed BTC intent advertisement.
• Add Alembic migration 016 for service heartbeats and update the facilitator verification script.

Why

Solana cannot safely share EVM signing assumptions, and native payment intents must not be advertised when their scanner is stale or unhealthy. This gives both rails explicit capability and readiness contracts.

Impact

Application defaults keep Solana disabled. Production activation requires an explicit Solana receiver, a configured network entry, CDP support with feePayer metadata, the companion web adapter, and the infrastructure rollout gate. BTC remains receive-only through an xpub and is advertised only after a fresh successful worker scan.

Zcash is intentionally excluded. The prior research PR #31 is deferred and closed.

Validation

• uv run ruff check .
• uv run mypy hyrule_cloud/ — 96 source files
• uv run pytest -q — 452 passed
• PYTHONPATH=. .venv/bin/alembic upgrade 015:016 --sql
• Focused payment, quote, intent, MCP, runtime, and launch-proof tests

Rollout

Merge with the companion web and infrastructure PRs, deploy with reviewed Vault values, and complete the paid EVM/Solana/BTC canary before declaring the rails promoted.

Companion PRs

• Web checkout: feat(payments): add Solana x402 checkout hyrule-web#39
• Production configuration and gates: feat(payments): gate Solana and BTC production rails network-operations#457
• Deferred Zcash research: [deferred] research: x402 Zcash binding #31