feat(core): add HashRateUnit string parsing

[cfilipescu]

Summary

• Adds string parsing support for HashRateUnit in the core type layer.
• Keeps unit conversion/parsing logic centralized so future API/config inputs can accept human-readable hashrate unit values safely.
• This is needed groundwork for upcoming integrations where units will be received as strings (for example from config, CLI, or external payloads).

[Copilot]

Pull request overview

Adds FromStr parsing for HashRateUnit in asic-rs-core, enabling conversion from common human-readable unit strings into the core unit enum to support future string-based inputs (config/CLI/external payloads).

Changes:

• Import std::str::FromStr in the hashrate core type module.
• Implement FromStr for HashRateUnit with normalization and multiple accepted aliases (e.g., TH, TH/s, TERAHASH).

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