test(json): pin s64 MIN/MAX writer bytes; move scratch to .sx-tmp

Close the coverage gap from attempt 1: example 0713 now builds integer
fields holding s64 MIN (-9223372036854775808) and s64 MAX
(9223372036854775807) — plus zero, a small negative, and a small positive —
and asserts the EXACT emitted bytes. This permanently pins the edge that
write_int is specifically engineered for (folding positives into negative
space so MIN's non-representable-positive magnitude serializes correctly).

s64 MIN is expressed as (0 - 9223372036854775807 - 1) because its magnitude
is not a representable positive s64 literal.

Test hygiene: stream to a repo-local, gitignored .sx-tmp/ path (created if
missing) instead of a fixed /tmp name, and unlink it right after read-back
so nothing leaks. Writer/model logic and src/ are untouched.

examples/expected/0713-modules-json-writer.stdout

@@ -1,4 +1,4 @@
-doc: {"name":"a\"b\n","tab":"x\ty","bs":"c\\d","ctrl":"\u0001","n":-7,"ok":true,"nil":null,"xs":[1,-2,3],"nested":{"k":"v"}}
+doc: {"name":"a\"b\n","tab":"x\ty","bs":"c\\d","ctrl":"\u0001","n":-7,"zero":0,"pos":7,"min":-9223372036854775808,"max":9223372036854775807,"ok":true,"nil":null,"xs":[1,-2,3],"nested":{"k":"v"}}
 buffer-exact: ok
 buffer-len: ok
 overflow-raised: ok