Apply multiple exact text replacements to a single existing file atomically.

Prefer `apply_patch` for most code/text edits when it is available, especially when a diff is easy to express. Use `multiedit` when you have several precise old/new replacements in one file, when a patch would be awkward, or after patch attempts fail.

Rules:
- Read the file first before editing.
- Each edit uses exact `oldString` -> `newString` replacement.
- All edits must succeed or none are written.
- Use `replaceAll: true` only when every occurrence should change.
