#515 scheme_exact_evm.md needs to specify encoding rules

[kladkogex]

Title Specify encoding rules for hex vs decimal in PaymentPayload

Description This PR updates scheme_exact_evm.md to clearly define the acceptable encoding formats for fields in PaymentPayload.

Specifically:

• signature, to and from are now explicitly defined as hex-encoded fields.
• nonce, value, validFrom, and validTo are defined as hex-encoded or decimal-encoded fields, depending on the prefix.

It is also specified, that leading zeroes may omitted for nonce, value, validFrom, and validTo.

This clarification ensures consistent interpretation of payloads across implementations and avoids ambiguity when encoding or decoding payment data.

Tests

• Verified consistency of PaymentPayload encoding with existing documentation examples.
• Manually reviewed all affected references in scheme_exact_evm.md for accuracy and clarity.
• Confirmed that example payloads follow the specified encoding rules.

Checklist

• [x] I have formatted and linted my code
• [x] All new and existing tests pass
• [x] My commits are signed (required for merge)

[cb-heimdall]

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