Writer ODText: support PreserveText fields

Problem

The ODText writer currently drops PHPWord PreserveText elements, including the page-number and other dynamic placeholders commonly used in headers and footers.

Expected behavior

Serialize PreserveText as native ODF content. Map supported placeholders such as {PAGE}, {NUMPAGES}, {DATE}, and {FILENAME} to ODF fields, preserve literal text and supported styling, and keep unknown placeholders literal instead of silently dropping them.

Scope

Implement this in the ODText writer using the existing PHPWord model and native ODF field elements. This is separate from the underlying ODText field support tracked in #2915.