fix(csv-stringify): preserve caller column definitions

[xiehuanyi]

Normalizing a column without header writes that field into the caller’s object. A frozen column therefore throws, and changing the key on a reused definition can keep the previous automatically inserted header.

Copy column definitions during normalization and apply the default header to the copy. Preserve the explicit key as well, including inherited, non-enumerable and getter-backed keys accepted by the existing API.

Validation:

• Regression cases cover frozen configuration, reuse after a key change, and three structural key definitions.
• Full csv-stringify suite: 237 passed, 1 existing pending test; TypeScript passed.
• Package build passed; full package lint passed with its existing pending-test warning.

Prepared with AI assistance. Source changes only; generated bundles were validated locally.

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