docs(period): fix #220 specify canonical u64 YYYYMM period format

[heqmah]

Closes #220

This Pull Request addes a strict period format contract to the repository documentation and interface definitions.

Context & Problem

Previously, integrations and references had inconsistencies regarding how time periods were represented. Some older scripts/documents referenced arbitrary string symbols such as dec2025, while testing frameworks and core logic used numeric representations (e.g. 202512). This lack of a canonical format caused confusion for external integrations, frontends, indexers, and developers designing transaction payloads.

Solution & Design

We formally document that the period parameter is defined as a u64 (unsigned 64-bit integer) representing calendar months using the strict YYYYMM convention (e.g. 202401 for January 2024, 202512 for December 2025).

The design includes:

1. Canonical Monthly Format: Specifies the exact numeric representation (YYYYMM), valid year range (2024-2100), and valid month range (01-12).
2. On-Chain Validation Mechanics: Explains how validation executes in $\mathcal{O}(1)$ time and space complexity using division (period / 100) and modulo (period % 100) arithmetic to save gas compared to string parsing.
3. Non-Monthly Period Handling: Explicitly details that non-monthly intervals (daily, weekly, quarterly) are not natively supported by the contract validation checks. To support them, integrations must map/bucket these intervals to a valid YYYYMM u64 value before generating cryptographic signatures and calling the contract.

This change ensures a single, unambiguous specification for both on-chain contract logic and off-chain indexer/relayer services.

Changed

• README.md : Updated the data model definition of period to specify the canonical u64 type, detailed the YYYYMM monthly validation rules, and added a section describing the mapping strategy for non-monthly periods.
• docs/signing-payload.md : Clarified the u64 cryptographic payload requirements and provided detailed mapping examples for quarterly, weekly, and daily wraps.
• src/lib.rs : Updated module-level Rust docstrings at the top of the file to formally document the period format contract.

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[heqmah]

@zintarh please kindly review