feat: configure records separator

[adrienaury]

No description provided.

[Copilot]

Pull Request Overview

This PR introduces a new records separator configuration and updates buffer handling to support locking based on a separator. Key changes include:

• Adding a new "locked" flag and separator handling to the Buffer struct and its methods.
• Updating the Buffer interface and configuration model to accept a separator.
• Refactoring the unfold and fold commands to use the new separator logic.

Reviewed Changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
pkg/posimap/core/buffer/buffer.go	Adds separator-based locking in buffer operations and updates Reset.
pkg/posimap/api/buffer.go	Updates the Buffer interface to include a separator parameter and a Fill method.
internal/appli/config/model.go	Introduces a new Separator field to the configuration model.
internal/appli/command/unfold.go	Refactors record creation and buffer resetting logic with separator support.
internal/appli/command/fold.go	Adapts fold command to use the new separator behavior.
internal/appli/charsets/utils.go	Adds helper functions for charset encoding.