feat: add cross-format integration tests and CLI end-to-end tests (#42)

- Add tests/cross_format.rs with 45 integration tests covering:
  - All format pair roundtrips: JSON↔YAML, JSON↔TOML, JSON↔CSV,
    YAML↔TOML, YAML↔CSV, TOML↔CSV
  - Three-format chains: JSON→YAML→TOML→JSON, YAML→TOML→JSON→YAML,
    JSON→CSV→YAML→JSON, CSV→JSON→TOML→CSV
  - Format-specific limitations: TOML top-level array rejection,
    TOML null→string conversion, CSV nested object flattening,
    CSV type inference, TOML datetime→string
  - Unicode cross-format roundtrips
  - Large dataset (500 rows) cross-format roundtrips
  - 11 CLI end-to-end tests: json→yaml, yaml→json, json→toml,
    csv→json, json→csv, --formats flag, file output, stdin→stdout,
    unknown format error, --pretty flag, --compact flag
- Fix CSV float serialization to preserve decimal point (87.0 not 87)

Fixes #14

Author: alvinreal

src/formats/csv.rs

@@ -206,7 +206,21 @@ fn csv_field_to_string(value: &Value) -> String {
         Value::Null => String::new(),
         Value::Bool(b) => b.to_string(),
         Value::Int(i) => i.to_string(),
-        Value::Float(f) => f.to_string(),
+        Value::Float(f) => {
+            let s = f.to_string();
+            // Ensure float representation always contains a decimal point
+            // so that CSV type inference re-parses it as Float, not Int.
+            if s.contains('.')
+                || s.contains('e')
+                || s.contains('E')
+                || s.contains("inf")
+                || s.contains("NaN")
+            {
+                s
+            } else {
+                format!("{s}.0")
+            }
+        }
         Value::String(s) => s.clone(),
         Value::Bytes(b) => format!("{b:?}"),
         Value::Array(_) | Value::Map(_) => {