fix(slashing): keep old-key signing info after rotation

[julienrbrt]

Description

Closes: #XXXX

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[giunatale]

Nitpick:

• The CHANGELOG entry is tagged (x/staking), but every code change in this PR is in x/slashing (hooks.go, signing_info.go); the tag should be (x/slashing).

Couple comments/issues:

• Shouldn't we eventually prune the old ValidatorSigningInfo, after it matures? Right now it's left indefinitely, and unnecessarily I might add, once the slashing window is elapsed, this entry is useless and just bloats state.
• double genesis-exported SigningInfo/ValidatorMissedBlocks entries carry byte-identical missed-block data for what is really one physical bitmap. This doesn't break re-import (InitGenesis re-applies the same bits idempotently), but it silently doubles genesis payload size per historical rotation and could double-count downtime if any tooling sums missed blocks across all exported entries.
• Optional: HandleValidatorSignature (from fix!: cherry-pick 19038 and add tests #108) resolves rotated addresses caller-side via ValidatorByConsAddr with no fallback, while this PR adds callee-side resolution with an old-address fallback (signingInfoAddr) for the jail/tombstone paths. Both land on the same record in every reachable state, but consider either reusing signingInfoAddr in HandleValidatorSignature or adding a short comment noting why the liveness path intentionally keeps strict no-fallback semantics, so the two mechanisms don't drift apart.

Also checked for logic bugs in the new address-resolution helpers (currentConsAddr/signingInfoAddr), consistency with the prior key-rotation fixes (#100, #108), genesis export/import behavior with the retained frozen records, and comment/behavior mismatches — no blocking issues found. The retained-record design and its rationale are clearly documented in the code and covered by the new TestHandleDoubleSign_AfterConsKeyRotation integration test.