format: a JSON document can be minified or indented

[donislawdev]

json gains one setting: formatting, taking record-per-line, minified
or indented. It defaults to the layout this format has always written, pinned
by two new golden values.

tfg generate --format json --size 1mb --set formatting=minified

The measurement that shaped this is a negative one

No reader can tell the three apart. The same records written four ways, on two
implementations in two languages:

layout	bytes	CPython json	V8 JSON.parse
minified	130	ok	ok
record-per-line	135	ok	ok
indented (2)	260	ok	ok
indented (4)	356	ok	ok

So the value of the setting sits entirely outside the parser. Measured on our
own 64 KB files: minified holds zero newlines and ends without one,
record-per-line holds 292, indented holds 3419 - and indented fits 201
records where record-per-line fits 290, so the same size carries a third less
data.

It also means the structural checker cannot work the layout out for itself, and
has to be told, for the reason the CSV dialect is told.

That falsified what the existing checker said about itself

check_json read "the manifest states one record per line" and counted lines
against it, so both new layouts would have broken it. The sentence was not wrong

• it stopped being true the day the layout became something a person can ask
  for. Second time in one session that a comment had to be rewritten rather than
  deleted for exactly this reason.

Each layout answers for its own floor

216 B minified, 219 B a record per line, 318 B indented. The registry
declares the default layout's floor - MinBytes is one number - and the
generator answers for the rest through SmallestAccepted, the same split as the
CSV dialect. The refusal names the layout it is about, because the same size is
legal in another one.

The canary

A checker handed a file and the right layout name would pass even if it
ignored the name entirely, so the guard hands it every wrong name instead.
Six pairs, six refusals.

Verification

• targeted guards green, gofmt, vet
• six mutations, all caught - including one already in the list that this
  refactor had staled, found by staleness.py rather than by reading
• 300 of 300 checks on the built binary, 15 of them new
• 12 documents read by the structural checker across the three layouts

The whole local suite was not run for this branch - CI covers it.

🤖 Generated with Claude Code