test(jsonc): kill mutation-survivors in jsonc + helper paths

[mcarvin8]

Close coverage gaps in disassemble.rs and reassemble.rs that the cargo-mutants baseline flagged as missed mutants. All assertions verify existing behavior; no production code changes.

disassemble.rs:

• unique_id_basename: cover the Value::Number and Value::Bool match arms (previously only Value::String was exercised), plus the fallthrough for missing / unsupported fields.
• sanitize: assert allowed-char preservation (-, _, ., alnum) and replacement of everything else with _, pinning each disjunct in the predicate.
• hash_string / hash_value: assert determinism, hex output, length, and input-sensitivity (pins both functions against constant-return mutants like Default::default() or "xyzzy").
• digit_width: assert the floor of 4 and the growth above 4-digit counts (kills /=→%=, +=→*=, and similar arithmetic mutants).
• leading_comment_start: cover start == 0 (kills >→>=, which loops forever via saturating_sub), consecutive comment walk-back, and stop-at-non-comment-line.
• line_end: pin pos + idx arithmetic and the no-newline fallback.
• jsonc_segment_with_comma: assert CRLF / LF stripping via trim_matches (kills ||→&&, which collapses the predicate).
• disassemble_file: assert pre-existing output is preserved when pre_purge = false (kills &&→|| on pre_purge && exists()).

reassemble.rs:

• leading_comment_start, line_end, jsonc_segment_with_comma: same kill-pattern coverage as the disassemble side.
• render_jsonc_property: assert CRLF normalization on the value.
• render_jsonc_array_element: assert the first line is not prefixed with a newline (kills >→>= on idx > 0).
• default_output_path: assert the path is built from Meta::source_filename with the requested output extension, and the fallback path when source_filename is None (kills the Ok(Default::default()) constant-return mutant).
• reassemble: round-trip through a nested target path to assert the parent directory is created (kills delete ! on the !parent.as_os_str().is_empty() guard).