[P1][M] Adapt zkx402 config attachment to x402 v2 route schema (accepts[])

[markballew]

Summary

Ensure zkx402 extensions (proofPolicy, proofCosts, access control) are attached consistently when x402 v2 uses accepts[] route definitions.

Acceptance criteria

• Decide and document where zkx402 config lives in v2:
  • per-route vs per-accept-option (and how conflicts are resolved)
• Demo policy/cost files still apply correctly (or migration path provided)
• Tests cover both discount tiers and hard gating under v2 route config