Reject out-of-range JSON numbers instead of silently serializing them as null

[LunaStev]

parse_number accepts the result of Rust's f64 parser without checking finiteness. A syntactically valid large exponent such as 1e9999 can produce infinity. The writer replaces non-finite Json::Num values with null, so parse/write can silently change a number into a different JSON type. The existing f64 representation needs a clear unsupported-range error rather than silent conversion.

Source evidence at the head of #520:

• utils/src/json.rs:220 — s.parse::<f64>()
• utils/src/json.rs:289 — if n.is_finite()

Acceptance:

• Reject a parsed non-finite numeric result with an actionable range error; a new arbitrary-precision dependency is not required.
• Cover positive/negative overflowing exponents and the largest supported finite values.
• Verify accepted finite number round trips remain numeric, including negative zero according to the current writer policy.
• Do not expand this patch into changing the public Json representation or all number formatting.

API/reference context: JSON permits implementations to bound numeric range, section 6.

Related: #351 is independent string-decoding work.

Audit status: identified by static source inspection; the scenarios above have not been executed during this audit. The permalink fixes the reviewed revision; this report does not claim the defect was introduced by #520.