/claim #164

PR Description

This PR starts the migration of destination-parameter functions to the rosenpass-to pattern as discussed in issue #164.

The scope of this change is intentionally small. The goal is to validate the migration approach on a single utility function before continuing with further refactors.

Scope

This PR converts b64_decode in util/src/b64.rs from a destination-parameter function:

rust fn b64_decode(input: &[u8], output: &mut [u8]) -> anyhow::Result<()>

to a function returning:

impl To<[u8], anyhow::Result<()>>

implemented using with_destination. All direct call sites were updated accordingly.

Changes Included

• Convert b64_decode to return impl To<[u8], anyhow::Result<()>>
• Preserve existing logic and error handling exactly
• Update direct call sites in:
• secret-memory
• wireguard-broker
• Update documentation examples to match the new usage pattern
• Add rosenpass-to dependency where required
• Remove unused import introduced during refactoring (anyhow::bail)

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Behavioral Changes

None.

This is a mechanical refactor only:

• no logic changes
• no allocation changes
• no performance changes
• no cryptographic code modified

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Pattern Followed

The refactor follows:

• the rosenpass-to pattern documented in /to/README.md
• existing usage already present in the repository
• the intended migration direction described in issue #164

No new abstractions or extension traits were introduced.

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Validation

The following checks were performed:

• cargo check succeeds for all affected crates
• all existing tests pass unchanged:
• rosenpass-util
• rosenpass-secret-memory
• rosenpass-wireguard-broker
• no new compiler warnings introduced by this change
• diff intentionally kept small for ease of review