Add 0715-modules-json-suite as the single comprehensive pinned suite for
std.json (mirrors 0711 for std.hash), alongside the focused 0713/0714 demos:
- ROUND-TRIP build->write->parse->write over a document covering EVERY value
kind (a string with every escape form \" \\ \b \f \n \r \t plus a \u00XX
control, integers 0 / negative / s64 MIN / s64 MAX, bool, null, array,
nested object) with insertion-order assertions, exact writer bytes, and
parse-then-rewrite idempotence.
- DECODE positives: \/, the full named-escape set, \uXXXX (BMP 1- and 2-byte)
plus a surrogate pair, the escaped control forms, and raw multi-byte UTF-8
round-tripping through writer + reader.
- MALFORMED matrix: one assertion per JsonParseError variant and its key
edges (UnexpectedToken, UnexpectedEnd, BadEscape, BadNumber incl. leading
zero / lone '-' / fraction / exponent / overflow, TrailingGarbage,
BadControlChar), each asserted to raise.
Pure test work: src/ and library/ untouched, no json.sx change needed. Every
model is built through an explicit Arena allocator (heap discipline).
