feat: add cross-format integration tests and CLI end-to-end tests

[alvinreal]

Summary

Closes #14 — Integration tests.

Changes

New test file: tests/cross_format.rs (45 integration tests)

Cross-format roundtrip tests (all pairs):

• JSON ↔ YAML (simple, nested, types with null, unicode, large 500-row dataset)
• JSON ↔ TOML (simple, nested, types, unicode)
• JSON ↔ CSV (array of objects, large dataset)
• YAML ↔ TOML (simple, nested, types, unicode)
• YAML ↔ CSV (both directions)
• TOML ↔ CSV (array of tables extraction)

Three-format chain tests (A→B→C→A):

• JSON → YAML → TOML → JSON
• YAML → TOML → JSON → YAML
• JSON → CSV → YAML → JSON
• CSV → JSON → TOML → CSV

Format limitation edge cases:

• TOML rejects top-level arrays
• TOML null becomes string "null"
• CSV flattens nested objects to JSON strings
• CSV type inference preserves int/float/bool on roundtrip
• TOML datetime becomes string in Value

CLI end-to-end tests (11):

• json→yaml, yaml→json, json→toml, csv→json, json→csv
• --formats flag, file output (-o), stdin→stdout
• Unknown format error, --pretty flag, --compact flag

Bug fix:

• CSV csv_field_to_string for Float now ensures decimal point is preserved (e.g., 87.0 instead of 87), so roundtripping floats through CSV maintains the Float type via type inference.

All checks pass: cargo build, cargo test (291 tests), cargo fmt --check, cargo clippy -D warnings.