feat: add streaming mode for large files

[alvinreal]

Adds a --stream CLI flag for element-by-element processing of large files, enabling morph to handle datasets that would otherwise require loading everything into memory.

New CLI Option

Flag	Description
--stream	Enable streaming mode (process elements one at a time)

Supported Streaming Pipelines

Input	Output	Streaming Type
JSONL	JSONL/JSON/CSV	True line-by-line (constant memory)
CSV	JSONL/JSON/CSV	True row-by-row (constant memory)
JSON array	JSONL/JSON/CSV	Parse + stream output

For unsupported combinations (e.g., YAML→JSON with --stream), the flag is silently ignored and the normal pipeline runs.

Implementation

• New src/streaming.rs module with:
  • StreamWriter — incremental output writer for JSONL, JSON arrays, and CSV
  • stream_jsonl() — line-by-line JSONL processing
  • stream_csv() — row-by-row CSV processing
  • stream_json_array() — JSON array streaming output
  • run_streaming() — full pipeline orchestrator
• Mapping support: per-element transforms work with -e/-m flags
• Format-specific options respected (e.g., --csv-delimiter in streaming mode)
• Output flushed via BufWriter for performance

Tests

• 24 unit tests in src/streaming.rs: JSONL/CSV/JSON streaming, mappings, edge cases, tab delimiters
• 7 integration tests in tests/cross_format.rs: CLI --stream flag with various format combinations

Fixes #29