fix: verify says whose the other files in the directory are

[donislawdev]

Stacked on #79, which is merged. This branch is rebased onto it.

A directory is allowed to hold more than one run - output.manifest exists so a
second run can record itself beside the first instead of being refused - and
verify called every file the neighbour had written extra, the word for a
file nobody asked for, and then called the directory a mismatch.

What was measured

Two runs one after another into one directory, name templates that do not
collide, both ending 0:

verify shared/manifest-alpha.json   ->  3 differences, exit 7
  extra b_0001.txt   extra b_0002.txt   extra manifest-beta.json
verify shared/manifest-beta.json    ->  4 differences, exit 7
  extra a_0001.txt   extra a_0002.txt   extra a_0003.txt   extra manifest-alpha.json

The report described a directory somebody had polluted. Every one of those files
was the neighbour's, and which neighbour was written down in the same directory
the whole time. The original report described this as being about the second
manifest - it is about every file of that run plus its manifest, and the
count depends on the neighbour's count.

cleanup needed nothing. It walks the manifest's entries rather than the
directory, so it has never known the word.

The repair

A new kind, another-run, carrying the name of the record that lists the file.
It does not count towards the verdict: a shared directory ends 0 and matched
is true. A real disagreement is untouched - missing or changed is still a
mismatch and still 7 - and the heading counts only the disagreements, so the
list under it does not mix the two.

This is the third category cut out of Extra for this reason, after
Leftover and Respelled, so the shape is the one already in the file.

Attribution, not suppression

Every file stays in the report. One entry each in --json, and in the prose one
line per neighbouring record rather than one per file:

note: manifest-big.json is another run's record. 10000 files here belong to it:
      bulk_0001.txt, bulk_0002.txt, bulk_0003.txt, and 9997 files not named here.

Untouchable rule 6 is kept literally rather than on trust, and a manifest
somebody leaves in a directory can claim a file out loud and cannot hide one.
That neighbour's ten thousand files used to print 10 001 lines and exit 7.

Two limits, both pinned by a guard

• A record is only opened when its name ends in .json. Reading the first bytes
  of every unlisted file was the first design and was measured out the same day.
  The factor is refused rather than reported: the canary runs of the
  unchanged binary disagreed with each other sevenfold while it was taken. What
  is not in doubt is that it was large enough to change the design.
• A file no manifest in the directory lists is still extra.

A second sieve was written and removed the same day. It read half a kilobyte
of a candidate looking for the first key of a manifest. Nothing could redden it -
a document that got past it was refused by the schema check behind it anyway -
so it changed no answer and no test could tell.

Cost

Interleaved with canaries, directory of ten thousand unlisted files:

	median	range
canary before	188 ms
before	228 ms	133 - 362 ms
after	258 ms	190 - 266 ms
canary after	194 ms

Canaries agree to 3%, and the two variants overlap, so no factor is claimed.

Guards

Seven guards, seven mutations, all caught. Two mutations did not compile at
first and were rewritten to consume what they replaced. staleness.py then
found five existing mutations that lifting the classification out of the
loop had invalidated.

Also in here, because the crowding ratchet caught it

• Three functions split out so the count of functions nesting three deep stays
  at 51 rather than being moved to 53. The ratchet only tightens.
• The comment on worstComplexity named rawTarget.resolveSize, and the
  guard's own log has named engine.Run since the run lock went in. The number
  has not moved, the holder has, and a comment naming the wrong function sends
  the next reader to flatten something that is not at the ceiling. O185
  records that Run now sits at exactly 22 of 22.

Full suite green, preflight --quick green.

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