Refactor: Extract CBT_Theme_Export service from rest_export_theme

[bph]

Part of #828 (which fulfills #688).

Goal

Same shape as the CBT_Theme_Save extraction, applied to the export flow. Move the orchestration out of rest_export_theme() into a new service class so it can be invoked from REST or CLI without duplication.

Scope

New file: includes/create-theme/theme-export.php

New class: CBT_Theme_Export with one public static method:

public static function to_zip( string $output_path, array $options ): true|WP_Error;

The current REST handler streams the zip back as the HTTP response body. The service writes to a path on disk; the REST wrapper writes to a temp file, streams it, then cleans up. This keeps both REST and CLI consumers symmetric.

REST endpoint update: rest_export_theme() becomes a thin wrapper. URL, permissions, and response semantics do not change.

Tests (PHPUnit, in this PR)

• Each export option (include images, export-ready, translate-ready) produces the expected zip contents
• Output path is honored
• Permission errors / disk failures propagate as WP_Error
• Test fixtures: a small block theme to round-trip

Out of scope

• Any CLI work — separate issue tracked in [Tracking] WP-CLI support: phased implementation #828
• Format options other than zip
• Changing the REST endpoint's semantics for existing callers

Acceptance

• Existing REST export behavior is byte-for-byte preserved (compare zip output before/after)
• lint:php, test:php, test:unit all green