chore: provide new_unchecked constructors for headers

[Fraser999]

Summary

• Add new_unchecked constructors to SignetHeaderV1 and SignetHeaderV2 that skip invariant validation, allowing infallible construction from trusted sources (e.g. headers already parsed and stored in the storage crates)
• Bump workspace version to 0.17.1

Motivation

Storage crates store plain alloy::consensus::Header values. When reading these back and wrapping them as signet header types, the validation in TryFrom<Header> is redundant - the data has already been validated on the way in. The new_unchecked constructors allow direct construction without re-checking invariants, which is both simpler (infallible) and avoids unnecessary work.

Test plan

• Existing signet-types tests pass (cargo t -p signet-types)
• Clippy clean with --all-features and --no-default-features
• Verify consuming crates can use new_unchecked to construct headers from stored data

[Fraser999]

• chore: provide new_unchecked constructors for headers #232 👈 (View in Graphite)
• main

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