fix(keystore): clamp unlock timeout and zeroize cached identities

[ouicate]

Summary

• Cap walletpassphrase unlock duration at 100,000,000 seconds, matching Bitcoin Core's walletpassphrase behavior.
• Enable x25519-dalek's zeroize feature so native age identity secret keys are overwritten when cached identities are dropped during walletlock or auto-relock.
• Replace bare Vec::clear() with clear_cached_identities() that drains identities and releases the Vec allocation via shrink_to_fit().

Addresses GHSA-49gv-v767-5q9w.

Motivation

KeyStore::unlock accepted timeout values up to u64::MAX, allowing RPC callers to disable auto-relock for effectively forever. Separately, age depends on x25519-dalek without the zeroize feature, so native identity secret keys were not guaranteed to be overwritten when identities were cleared from the in-memory cache.

[ouicate]

Please review @nullcopy

[nullcopy]

Thanks for the suggestions. The ideas are good, but I would prefer different solutions.

1. capping the timeout to 100K secs to match Bitcoin. This does prevent a panic if the caller specifies an unlock time large enough to trigger overflow. Rather than silently falling back to an unlock time the caller did not request, I would prefer to just do checked_add() and return an error to the caller if their requested unlock time is too large, and allow them to try again with a smaller value.

2. the helper to clear cached identities is unnecessary. Simply enabling the zeroized feature accomplishes the stated goal.

[nullcopy]

Superseded by #641