F2.3: pin std.json round-trip + malformed-input suite (examples/0715)

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).

examples/expected/0715-modules-json-suite.exit

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