Fill the highest-value coverage gaps found in the Codable bridge and
parser: the full integer-width decode ladder and per-width out-of-range
errors, Float round-trip, NaN equality and hash consistency (the bespoke
nan == nan invariant), unkeyed decodeNil false-branch and nested-index
mismatch, and the previously untested expectedNode, unterminated flow
mapping, explicit block-scalar under-indent, and buffered invalid-merge
error paths.
