test: cover surviving mutants in libstackerdb + stacks-codec

[simone-stacks]

Three targeted unit tests that kill 14 surviving cargo-mutants:

• StackerDBChunkData::{sign,verify} round-trip — closes the verify -> Ok(true) mutant (vacuous signature acceptance).
• StackerDBChunkData consensus codec round-trip — closes the no-op consensus_serialize mutant.
• read_next_at_most / read_next_exact length-bound tests in stacks-codec (which shipped with zero unit tests) — closes 11 mutants on the consensus-wire length-prefix guards.

Result of a first smoke-test of the quality-agent.

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