Review feedback on #2879, citing rules/common/coding-style.md ("Immutability
(CRITICAL): ALWAYS create new objects, NEVER mutate existing ones"). Build one
list of locale entries, then derive the matched and orphaned sets from it with
filter/map, and build the canonical Map the same way.
Behaviour is unchanged and both mutations still fail: reintroducing the drift in
one locale file gives `Passed: 4, Failed: 3`, an orphan gives `Passed: 6,
Failed: 1`, both exit 1.
Nguyen Thanh Dat
5f79c39687