Exercise 2: FileWatcher Distinguishing Between Two Real Files — Possible Solution
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THE SETUP
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    file_a = 'watch_multi_test/a.html'   # real file, written with 'a v1'
    file_b = 'watch_multi_test/b.html'   # real file, written with 'b v1'

    watcher = FileWatcher([file_a, file_b])

VERIFIED, REAL RESULTS
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    watcher.changed()                          # None -- nothing edited yet

    # edit ONLY file_a (write 'a v2')
    watcher.changed()                          # file_a's own real path
    watcher.changed() == file_a                # True

    watcher.changed()                          # None again -- already consumed

    # edit ONLY file_b (write 'b v2')
    watcher.changed()                          # file_b's own real path
    watcher.changed() == file_b                # True

WHY THIS WORKS AS AN ANSWER
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FileWatcher.changed() loops over self.paths in order and returns the
first path whose real, freshly-read mtime differs from what was
stored, updating that one entry in self._mtimes before returning -
file_b's own stored mtime is never touched while only file_a has
changed, so it correctly stays silent about file_b until file_b is
genuinely edited too. Verifying both files independently, with a
"nothing changed" check in between, confirms the watcher tracks each
real file's own state separately rather than collapsing "something
changed" into one shared flag.