Function verifyChain

Verify the entire audit trail chain.

Walks from the first event to the last, verifying:

1. First event has prevHash === GENESIS_HASH
2. Each subsequent event has prevHash === previous event's chainHash
3. Each event's chainHash matches recomputation

Events without chainHash/prevHash fields are skipped (legacy support). The chain continues from the last known hash after skipped events.

In strict mode (options.strict = true), skipped events make the chain invalid. Regulated verification paths must use strict mode.