rename the companion-type suffix from Json to Data, and stop recognizing the old one

[eburgos]

A declaration named XJson published every generated name as X — the
TypeScript type, the Zod consts, the JSON Schema id, and what a reference to it
resolves as. The recognized suffix is Data now, with the same mechanics:
suffix-only, exact case, and XData publishes as X everywhere a name is
read. Json is not recognized at all — a hard switch, taken after auditing the
consuming workspace and finding no Json-suffixed declaration to migrate.

Recognition lives in one place where it lived in two. The reference-resolution
arm was behaviourally identical to the stripping seam once the guard sits
there, so it now routes through the same function instead of repeating the
suffix, and a grep finds no other name-level suffix reading.

Stripping never empties a name: a type named exactly Data keeps it. The old
code lacked that guard — Json alone stripped to the empty string — so it is
new, not preserved.

Pinned from both directions: SomethingData publishes as Something on all
three surfaces including reference resolution, and SomethingJson publishes
as itself, so the old convention is provably gone rather than merely
undocumented. Eleven fixtures migrated to the new suffix; one incidental
Json-named fixture about JSON Schema itself was respelled to match its
siblings; the docs' naming-convention sections and worked examples move with
it.

just fmt, just check, just quick, doctests, just lint-all across all 68
toggles, and the test powerset across all 68 in four partitions — all green.