feat: add ICP chain specification for `exact` scheme

[zensh]

Description

Document the exact payment scheme for x402 on ICP, including payment requirements, X-PAYMENT header schema, signing flow, and verification/settlement guidance.

Tests

Not applicable (documentation only)

Checklist

• [x] I have formatted and linted my code
• [x] All new and existing tests pass
• [x] My commits are signed (required for merge) -- you may need to rebase if you initially pushed unsigned commits

[cb-heimdall]

🟡 Heimdall Review Status

Requirement	Status	More Info
Reviews	🟡 0/1	Denominator calculation Show calculation 1 if user is bot 0 1 if user is external 0 2 if repo is sensitive 0 From .codeflow.yml 1 Additional review requirements Show calculation Max 0 0 From CODEOWNERS 0 Global minimum 0 Max 1 1 1 if commit is unverified 1 Sum 2

[vercel[bot]]

@zensh is attempting to deploy a commit to the Coinbase Team on Vercel.

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