JSON and NDJSON input and output format support

[vmvarela]

Part of #68
Depends on #95 (format plugin architecture)

Description

Add JSON array-of-objects and newline-delimited JSON (NDJSON) as input and output formats. This generalizes the existing --json output flag into the new --output-format json system, and adds JSON as an input source.

Acceptance Criteria

• --input-format json reads a JSON array of objects (each object becomes a row)
• --input-format ndjson reads one JSON object per line
• --output-format json produces a JSON array of objects (column names as keys)
• --output-format ndjson produces one JSON object per line
• Nested JSON objects in input are serialized as JSON strings (not flattened)
• JSON arrays in input are serialized as JSON strings
• Type detection: numbers, booleans, and null values are handled correctly
• Malformed JSON produces a clear error with line number

Notes

• JSON input flattening strategy: top-level keys only; nested values become JSON strings
• The existing --json flag should map to --output-format json internally after Format plugin architecture for multi-format I/O #95
• Consider using Zig's std.json for parsing