a content class is worked out once, not once per copy

Lookup walked the whole size group for every file in it, so N copies of
one file cost N walks of an N-member group, each taking the index's
lock at every step - which is why more workers made it slower rather
than faster. Every member of a class elects the same original (the
invariant identicalTo already documents and a test already pins), so
the class is memoised on the first walk and every later member is a
map lookup.

Measured over identical files, invented data, same machine:

  500 files   0.17s -> 0.12s
  2000 files  1.89s -> 0.58s

and the plans are byte-identical once the sandbox path is normalised.
The test counts walks: one per content class, not one per file.

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