fix(cli): compact nested raw JSON values into single-line records (FUZZ-017 r2)

[randlee]

Fixes comp-t-7.fuzz-017-r2-fix (round 2 of FUZZ-017, filed by comp-t-7.6-qa): render --append compacts insignificant whitespace in nested raw JSON values so every record is one physical line, without re-parsing number or string lexemes. Regression test asserts one line per record for a nested value with spaces and newlines.

Rebased b5a181f -> b1cad13 onto #716. Gates at b1cad13: clippy -D warnings clean, fmt clean, cargo test --workspace 1064 passed / 0 failed / 0 ignored.

🤖 Generated with Claude Code

[randlee]

QA fix verification: comp-t-7.fuzz-017-r2-qa (fuzz-017) at b1cad13

Verdict: PASS. 1 carried finding verified fixed (round 2).

Finding	Disposition
fuzz-017	fixed

The regression found in round 1 is gone. The fuzz coordinator, locked to fuzz-017, verified at the pinned commit:

• The reproducer body appends exactly one physical line that parses as JSON ({"a":{"x":1,"y":[1,2]},"b":3}; round 1 gave 3 lines). Tab and CRLF between tokens also give one line.
• Numbers stay as spelled (12345678901234567890123, 0.1000000000000000055511151231257827, 1e2, 1E+2, -0, -0.0), and string contents (spaces, escaped \n/\t, braces, \", \\, \u escapes) are byte-identical to round 1.
• Three appends give three lines, no blank lines, one trailing newline. Malformed and non-object bodies still fail closed with the file byte-identical. Compact bodies are byte-identical to round 1.
• cargo test -p sc-compose --test cli fuzz_017: 2 passed; the full sc-compose suite passes. The new test fails at the round-1 commit ebe5e56.
• Local fmt, clippy -D warnings and tests are clean at this head.

Notes: object keys are re-serialized by serde, so a key written as ék comes out as ék, and keys are sorted with duplicates resolved to the last value. Both are the same as round 1; I did not compare them with the pre-fix baseline binary.

Disclosures: CI not checked (rate-limit rule). The fix-round scope helper accepted the coordinator as the filing reviewer.